Theory of the Earth Volume I Part 16

The fact is certain, that those alpine bodies are much harder, or less subject to dissolution and decay, than the horizontal strata. But this must be taken in the general, and will by no means apply to particular cases which might be compared. Nothing, for example, more solid than the lime-stones, or marbles, and iron-stones; nothing more hard or solid than the chirt or flint; and all these are found among the horizontal strata. But, while some strata among those horizontal beds are thus perfectly solid, others are found with so slight degrees of consolidation, that we should not be able to ascribe it to the proper cause, without that gradation of the effect, which leads us to impute the slightest degree of consolidation to the same operations that have produced the complete solidity. While, therefore, the most perfect solidity is found in certain strata, or occasionally among the horizontal bodies, this forms no part of their character in general, or cannot be considered as a distinctive mark, as it truly is with regard to the alpine strata. These last have a general character of consolidation and indissolubility, which is in a manner universal. We are, therefore, now to inquire into the cause of this distinction, and to form some hypothesis that may be tried by the actual state of things, in being compared with natural appearances.

As the general cause of consolidation among mineral bodies, formed originally of loose materials, has been found to consist in certain degrees of fusion or cementation of those materials by means of heat; and as, in the examination of the horizontal strata we actually find very different degrees of consolidation in the several strata, independent of their positions in relation to height or depth, we have reason to believe that the heat, or consolidating operation, has not been equally employed in relation to them all.

We are not now inquiring how an inferior stratum should have been heated in a lesser degree, or not consolidated, while a superior stratum had been consolidated in the most perfect manner; we are to reason upon a fact, which is, that the horizontal strata in general appear not to have been equally or universally consolidated; and this we must attribute to an insufficient exertion of the consolidating cause. But, so far as the erecting cause is considered as the same with that by which the elevated bodies were consolidated, and so far as the vertical situation is a proof of the great exertion of that subterraneous power, the strata which are most erected, should in general be found most consolidated.

Nothing more certain than that there have been several repeated operations of the mineralising power exerted upon the strata in particular places; and all those mineral operations tend to consolidation: Therefore, the more the operations have been repeated in any place, the more we should find the strata consolidated, or changed from their natural state. Vertical strata have every appearance from whence we should be led to conclude, that much of the mineral power had been exerted upon them, in changing their original const.i.tution or appearance. But the question now to be considered is this, How far it may appear that these ma.s.ses of matter, which now seem to be so different from the ordinary strata of the globe, had been twice subjected to the mineral operations, in having been first consolidated and erected into the place of land, and afterwards sunk below the bottom of the sea, in order a second time to undergo the process of subterraneous heat, and again be elevated into the place where they now are found.

It must be evident, here is a question that may not be easy to decide.

It is not to the degree of any change to which bodies may be subject, that we are to appeal, in order to clear up the point in question, but to a regular course of operations, which must appear to have been successively transacted, and by which the different circ.u.mstances or situations of those ma.s.ses are to be discovered in their present state.

Now, though it does not concern the present theory that this question be decided, as it is nothing but a repet.i.tion of the same operations that we look for; nevertheless, it would be an interesting fact in the natural history of this earth; and it would add great l.u.s.tre to a theory by which so great, so many operations were to be explained. I am far from being sanguine in my expectations of giving all the satisfaction in relation to this subject that I could wish; but it will be proper to state what I have lately learned with regard to so curious a question, that others, who shall have the opportunity, may be led to inquire, and that thus the natural history of the earth may be enlarged, by a proper investigation of its mineral operations.

With this view I have often considered our schistus mountains, both in the north and south; but I never found any satisfactory appearance from whence conclusions could be formed, whether for the question or against it. The places I examined were those between the alpine countries and the horizontal strata; here, indeed, I have frequently found a confused ma.s.s, formed of the fragments of those alpine strata mixed with the materials of the horizontal bodies; but not having seen the proper shape and connection of those several deposits, I always suspended my judgment with regard to the particular operations which might have been employed in producing those appearances.

I had long looked for the immediate junction of the secondary or low country strata with the alpine schistus, without finding it; the first place in which I observed it was at the north end of the island of Arran, at the mouth of Loch Ranza; it was upon the sh.o.r.e, where the inclined strata appeared bare, being; washed by the sea. It was but a very small part that I could see; but what appeared was most distinct.

Here the schistus and the sandstone strata both rise inclined at an angle of about 45; but these primary and secondary strata were inclined in almost opposite directions; and thus they met together like the two sides of a _lambda_, or the rigging of a house, being a little in disorder at the angle of their junction. From this situation of those two different ma.s.ses of strata, it is evidently impossible that either of them could have been formed originally in that position; therefore, I could not here learn in what state the schistus strata had been in when those of the sand-stone, &c, had been superinduced.

Such was the state of my mind, in relation to that subject:, when at Jedburgh upon a visit to a friend, after I had returned from Arran, and wrote the history of that journey; I there considered myself as among the horizontal strata which had first appeared after pa.s.sing the Tweed, and before arriving at the Tiviot. The strata there, as in Berwickshire, which is their continuation to the east, are remarkably horizontal for Scotland; and they consist of alternated beds of sand-stone and marl, or argillaceous and micaceous strata. These horizontal strata are traversed in places with small veins of whin-stone, as well as greater ma.s.ses forming rocks and hills of that material; but, except it be these, (of which there are some curious examples), I thought there could be nothing more of an interesting nature to observe. Chance, however, discovered to me what I could not have expected or foreseen.

The river Tweed, below Melrose, discovers in its bed the vertical strata of the schistus mountains, and though here these indurated bodies are not veined with quartz as in many places of the mountains, I did not hesitate to consider them as the same species, that is to say, the marly materials indurated and consolidated in those operations by which they had been so much changed in their place and natural position. Afterwards in travelling south, and seeing the horizontal softer strata, I concluded that I had got out of the alpine country, and supposed that no more of the vertical strata were to be observed.

The river Tiviot has made a wide valley as might have been expected, in running over thole horizontal strata of marly or decaying substances; and the banks of this river declining gradually are covered with gravel and soil, and show little of the solid strata of the country. This, however, is not the case with the Jed, which is to the southward of the Tiviot; that river, in many places, runs upon the horizontal strata, and undermines steep banks, which falling shows high and beautiful sections of the regular horizontal strata. The little rivulets also which fall into the Jed have hollowed out deep gullies in the land, and show the uniformity of the horizontal strata.

In this manner I was disposed to look for nothing more than what I had seen among those mineral bodies, when one day, walking in the beautiful valley above the town of Jedburgh, I was surprised with the appearance of vertical strata in the bed of the river, where I was certain that the banks were composed of horizontal strata. I was soon satisfied with regard to this phenomenon, and rejoiced at my good fortune in stumbling upon an object so interesting to the natural history of the earth, and which I had been long looking for in vain.

Here the vertical strata, similar to those that are in the bed of the Tweed, appear; and above those vertical strata, are placed the horizontal beds, which extend along the whole country.

The question which we would wish to have solved is this; if the vertical strata had been broken and erected under the superinc.u.mbent horizontal strata; or if, after the vertical strata had been broken and erected, the horizontal strata had been deposited upon the vertical strata, then forming the bottom of the sea. That strata, which are regular and horizontal in one place, should be found bended, broken, or disordered at another, is not uncommon; it is always found more or less in all our horizontal strata. Now, to what length this disordering operation might have been carried, among strata under others, without disturbing the order and continuity of those above, may perhaps be difficult to determine; but here, in this present case, is the greatest disturbance of the under strata, and a very great regularity among those above. Here at least is the most difficult case of this kind to conceive, if we are to suppose that the upper strata had been deposited before those below had been broken and erected.

Let us now suppose that the under strata had been disordered at the bottom of the sea, before the superinc.u.mbent bodies were deposited; it is not to be well conceived, that the vertical strata should in that case appear to be cut off abruptly, and present their regular edges immediately under the uniformly deposited substances above. But, in the case now under consideration, there appears the most uniform section of the vertical strata, their ends go up regularly to the horizontal deposited bodies. Now, in whatever state the vertical strata had been in at the time of this event, we can hardly suppose that they could have been so perfectly cut off, without any relict being left to trace that operation. It is much more probable to suppose, that the sea had washed away the relics of the broken and disordered strata, before those that are now superinc.u.mbent had been begun to be deposited. But we cannot suppose two such contrary operations in the same place, as that of carrying away the relics of those broken strata, and the depositing of sand and subtile earth in such a regular order. We are therefore led to conclude, that the bottom of the sea, or surface of those erected strata, had been in very different situations at those two periods, when the relics of the disordered strata had been carried away, and when the new materials had been deposited.

If this shall be admitted as a just view of the subject, it will be fair to suppose, that the disordered strata had been raised more or less above the surface of the ocean; that, by the effects of either rivers, winds, or tides, the surface of the vertical strata had been washed bare; and that this surface had been afterwards sunk below the influence of those destructive operations, and thus placed in a situation proper for the opposite effect, the acc.u.mulation of matter prepared and put in motion by the destroying causes.

I will not pretend to say that this has all the evidence that should be required, in order to const.i.tute a physical truth, or principle from whence we were to reason farther in our theory; but, as a simple fact, there is more probability for the thing having happened in that manner than in any other; and perhaps this is all that may be attained, though not all that were to be wished on the occasion. Let us now see how far any confirmation may be obtained from the examination of all the attending circ.u.mstances in those operations.

I have already mentioned, that I had long observed great ma.s.ses of _debris_, or an extremely coa.r.s.e species of pudding-stone, situated on the south as well as north sides of those schistus mountains, where the alpine strata terminate in our view, and where I had been looking for the connection of those with the softer strata of the low country.

It has surely been such appearances as these which have often led naturalists to see the formation of secondary and tertiary strata formed by the simple congestion of _debris_ from the mountains, and to suppose those ma.s.ses consolidated by the operation of that very element by which they had been torn off from one place and deposited in another. I never before had data from whence to reason with regard to the natural history of those ma.s.ses of gravel and sand which always appeared to me in an irregular shape, and not attended with such circ.u.mstances as might give light into their natural history; but now I have found what I think sufficient to explain those obscure appearances, and which at the same time will in some respect ill.u.s.trate or confirm the conjecture which has now been formed with regard to the operations of the globe in those regions.

In describing the vertical and horizontal strata of the Jed, no mention has been made of a certain pudding-stone, which is interposed between the two, lying immediately upon the one and under the other. This puddingstone corresponds entirely to that which I had found along the skirt of the schistus mountains upon the south side, in different places, almost from one end to the other. It is a confused ma.s.s of stones, gravel, and sand, with red marly earth; these are consolidated or cemented in a considerable degree, and thus form a stratum extremely unlike any thing which is to be found either above or below.

When we examine the stones and gravel of which it is composed, these appear to have belonged to the vertical strata or schistus mountains.

They are in general the hard and solid parts of those indurated strata, worn and rounded by attrition; particularly sand or marl-stone consolidated and veined with quartz, and many fragments of quartz, all rounded by attrition. In this pudding-stone of the Jed, I find also rounded lumps of porphyry, but have not perceived any of granite.[32]

This however is not the case in the pudding-stone of the schistus mountains, for, where there is granite in the neighbourhood, there is also granite in the pudding-stone.

[Note 32: A view of this object is seen in plate 3d. It is from a drawing taken by Mr Clerk of Eldin.]

From this it will appear, that the schistus mountains or the vertical strata of indurated bodies had been formed, and had been wasted and worn in the natural operations of the globe, before the horizontal strata were begun to be deposited in those places; the gravel formed of those indurated broken bodies worn round by attrition evince that fact. But it also appears that the mineral operations of the globe, melting and consolidating bodies, had been exerted upon those deposited strata above the vertical bodies.

This appears evidently from the examination of our pudding-stone. The vertical strata under it are much broken and injected with ferruginous spar; and this same spar has greatly penetrated the pudding-stone above, in which are found the various mineral appearances of that spar and iron ore.

But those injecting operations reach no farther up among the marl strata in this place; and then would appear to have been confined to the pudding-stone. But in another place, about half a mile farther up the river, where a very deep section of the strata is discovered, there are two injections from below; the one is a thin vein of whin-stone or basaltes, full of round particles of steat.i.tes impregnated with copper; it is but a few inches wide, and proceeds in a kind of zigzag. The other appears to have been calcareous spar, but the greatest part of it is now dissolved out. The strata here descend to the bottom of the river, which is above the place of the pudding-stone and vertical strata. Neither are these last discoverable below the town of Jedburgh, at least so far as I have seen; and the line of division, or plane of junction of the vertical and horizontal strata, appears to decline more than the bed of the river.

But it may be asked, how the horizontal strata above, among which are many very strong beds, have been consolidated. The answer to this question is plain. Those strata have been indurated or consolidated in no other manner than the general strata of the earth; these being actually the common strata of the globe; while the vertical or schistus strata are the ordinary strata still farther manufactured, (if we may be allowed the expression) in the vicissitude of things, and by the mineral operations of the globe. That those operations have been performed by subterraneous heat has been already proved; but I would now mention some particular appearances which are common or general to those strata, and which can only be explained upon that principle.

The red marly earth is prevalent among those strata; and it is with this red ferruginous substance that many of the sand-stone strata are tinged.

It is plain that there had been an uniform, deposits of that sand and tinging earth; and that, however different matter might be successively deposited, yet that each individual stratum should be nearly of the same colour or appearance, so far as it had been formed uniformly of the same subsiding matter. But, in the most uniform strata of red sand-stone, the fracture of the stone presents us with circular spots of a white or bluish colour; those little spheres are in all respects the same with the rest of the stone, they only want the tinging matter; and now it may be inquired how this has come about.

To say that sphericles of white sand should have been formed by subsiding along with the red sand and earth which composed the uniform stratum whether of sand-stone or marl, (for it happens equally in both,) is plainly impossible, according to our notion of that operation in which there is nothing mysterious. Those foliated strata, which are of the most uniform nature, must have been gradually acc.u.mulated from the subsiding sand and earth; and the white or colourless places must have had their colour destroyed in the subsequent cementing operations. It is often apparent, that the discharging operation had proceeded from a centre, as some small matter may be perceived in that place. I know not what species of substance this has been, whether saline or phlogistic, but it must have had the power of either volatilising or changing the ferruginous or red tinging substance so as to make it lose its colour.

I have only mentioned spherical spots for distinctness sake; but this discharging operation is found diversifying those strata in various ways, but always referable to the same or similar causes. Thus, in many of the veins or natural cracks of those strata, we find the colour discharged for a certain s.p.a.ce within the strata; and we often see several of those spots united, each of them having proceeded from its own centre, and uniting where they approached. In the two veins above mentioned, of whin-stone and spar traversing the strata, the colour of the strata is, discharged more or less in the places contiguous with the veins.

I am now to mention another appearance of a different kind. Those strata of marl are in general not much consolidated; but among, them there are sometimes found thin calcareous strata extremely consolidated, consequently much divided by veins. It is in the solid parts of those strata, perfectly disconnected from the veins, that there are frequent cavities curiously lined with crystals of different sorts, generally calcareous, sometimes containing also those that are siliceous, and often accompanied with pyrites. I am persuaded that the origin of those cavities may have been some hollow sh.e.l.ls, such as _echini_ or some marine object; but that calcareous body has been so changed, that it is not now distinguishable; therefore, at present, I hold this opinion only as conjecture.

Having, in my return to Edinburgh, traveled up the Tiviot, with a view to investigate this subject of primary and secondary operations of the earth, I found the vertical strata, or alpine schistus, in the bed of the river about two miles below Hawick. This was the third time I had seen those vertical bodies after leaving the mountains of Lauderdale.

The first place was the bed of the river Tweed, at the new bridge below Melrose; but here no other covering is to be seen above those vertical strata besides the soil or traveled earth which conceals every thing except the rock in the bed of the river. The second place was Jedburgh, where I found the vertical strata covered with the horizontal sandstone and marl, as has been now described. The third place was the Tiviot, and this is that which now remains to be considered.

Seeing the vertical strata in the bed of the river, I was desirous to know if those were immediately covered with the horizontal strata. This could not be discovered in the bed of the river where the rock was covered upon the banks with travelled earth. I therefore left the river, and followed the course of a brook which comes from the south side. I had not gone far up the bank, or former boundary of the Tiviot, when I had the satisfaction to find the vertical strata covered with the pudding-stone and marly beds as in the valley of the Jed.

It will now be reasonable to suppose that all the schistus which we perceive, whether in the mountains or in the valleys, exposed to our view had been once covered with those horizontal strata which are observed in Berwickshire and Tiviotdale; and that, below all those horizontal strata in the level country, there is at present a body or basis of vertical or inclined schistus, on which the horizontal strata of a secondary order had been deposited. This is the conclusion that I had formed at Jedburgh, before I had seen the confirmation of it in the Tiviot; it is the only one that can be formed according to this view of things; and it must remain in the present state until more evidence be found by which the probability may be either increased or diminished.

Since writing this, I have read, in the Esprit de Journaux, an abstract of a memoir of M. Voigt, upon the same subject, which I shall now transcribe.

"La mer a commence par miner les montagnes primitives dont les debris se sont precipites au fond. Ces debris forment la premiere couche qui est posee immediatement sur les montagnes primitives. D'apres l'ancien langage de mineurs, nous avons jusqu'aujourd'hui appelle cette couche _le sol mort rouge_, parce qu'il y a beaucoup de rouge dans son melange, qu'elle forme le sol ou la base d'autres couches, et peut-etre de toutes, qu'elle est entierement inutile et, en quelque facon, morte pour l'exploitation des mines. Plusieurs se sont efforces de lui donner un nom harmonieux; mais ils ne l'ont pu sans occasionner des equivoques.

Les mots _Breche Puddinstone Conglomerations_, &_c_. designent toujours des substances autres que cette espece de pierre.

"Il est tres agreable de l'examiner dans les endroits ou elle forme des montagnes entieres. Cette couche est composee d'une quant.i.te prodigieuse de pierres arrondies, agglutinees ensemble par une substance argileuse rouge et meme grise, et le toute a acquis a.s.sez de durete. On ne trouve dans sa composition aucune espece de pierre qui, a en juger par les meilleures observations, puisse avoir ete formee plus tard qu'elle; on n'y voit par-tout que des parties et des produit des montagnes primitives princ.i.p.alement de celles qui abondent le plus dans ces contrees. Le sol mort, par exemple, qui compose les montagnes des environs de Walbourg, pres d'Eisenach, contient une quant.i.te de gros morceaux de granit et de schiste micace; c'est vraisemblablement parce que les montagnes primitives les plus voisines de Rhula, etc. sont, pour la plus part, formees de ces deux especes de pierres. Pres de Goldlauter, le sol mort consiste presque tout en porphyre, substance dont sont formees les montagnes primitives qui y dominent; et le Kiffauserberg dans la Thuringe a probablement recu ces morceaux arrondis de schiste argileux des montagnes voisine du Hartz. Vous trouverez ici que le schiste argileux existoit deja lorsque la mer a jette les premiers fondemens de nos montagnes stratifiees. Je serois fort etonne que quelqu'un me montrat un sol mort qui contint un morceaux de gypse, de marne, de pierre puante et autres. Quoiqu'il en soit il n'est pas aise d'expliquer pourquoi on ne trouve point de corps marins petrifies dans cette espece de pierre. C'est peut-etre que, par l'immense quant.i.te de pierres dures roulees dans le fond de la mer, ils ont ete brises avant qu'ils aient commence de s'agglutiner ensemble. Mais on rencontre sur-tout au Kiffhauserberg des troncs d'arbres entiers petrifies; preuve qu'il y avoit deja ou de la vegetation avant que l'ocean destructeur se fut empare de ces cantons, ou du moins que quelques isles avoient existe au-dessus de la surface."

Here we find the same observations in the mountains of Germany that I have been making with regard to those of Scotland. I have formerly observed ma.s.ses of the same kind in the west of England, to the east of the Severn; but I could not discover any proper connection of that ma.s.s with the regular strata. I have also long observed it in many parts of Scotland, without being able to attain a sufficiently satisfactory idea with regard to those particulars by which the alternation of land and water, of the superficial and internal mineral operations of the globe, might be investigated.

It will be very remarkable if similar appearances are always found upon the junction of the alpine with the level countries. Such an appearance, I am inclined to think, may be found in the Val d'Aoste, near Yvree. M.

de Saussure describes such a stone as having been employed in building the triumphal arch erected in honour of Augustus. "Cet arc qui etoit anciennement revetu de marbre, est construit de grands quartiers d'une espece a.s.sez singuliere de poudingue ou de gres a gros grains. C'est une a.s.semblage de fragmens, presque touts angulaires, de toutes sortes de roches primitives feuilletees, quartzeuses, micacees; les plus gros de ces fragmens n'atteignent pas le volume, d'une noisette. La plupart des edifices antiques de la cite l'Aoste et de ses environs, sont construits de cette matiere; et les gens du pays sont persuades que c'est une composition; mais j'en ai trouve des rochers en place dans les montagnes au nord et au-dessus de la route d'Yvree."

We may now come to this general conclusion, that, in this example of horizontal and posterior strata placed upon the vertical _schisti_ which are prior in relation to the former, we obtain a further view into the natural history of this earth, more than what appears in the simple succession of one stratum above another. We know, in general, that all the solid parts of this earth, which come to our view, have either been formed originally by subsidence at the bottom of the sea, or been transfused in a melted state from the mineral regions among those solid bodies; but here we further learn, that the indurated and erected strata, after being broken and washed by the moving waters, had again been sunk below the sea, and had served as a bottom or basis on which to form a new structure of strata; and also, that those new or posterior strata had been indurated or cemented by the consolidating operations of the mineral region, and elevated from the bottom of the sea into the place of land, or considerably above the general surface of the waters.

It is thus that we may investigate particular operations in the general progress of nature, which has for object to renovate the surface of the earth necessarily wasted in the operation of a world sustaining plants and animals.

It is necessary to compare together every thing of this kind which occurs; it is first necessary to ascertain the fact of their being a prior and posterior formation of strata, with the mineral operations for consolidating those bodies formed by collection of the moveable materials; and, secondly, it is interesting to acquire all the data we can in order to form a distinct judgment of that progress of nature in which the solid body of our land is alternately removed from the bottom of the sea into the atmosphere, and sunk again at the bottom of the sea.

I shall now transcribe what M. Schreiber has wrote in relation to this subject. It is in a memoir concerning the gold mine of Gardette, published in the Journal de Physique.

"Avant de quitter la montagne de la Gardette qu'il me soit permis de rapporter une observation qui peut-etre n'est pas denuee de tout interet pour les naturalistes; je l'ai faite dans une galerie a environ cinquante-trois toises a l'ouest du princ.i.p.al puit laquelle a ete poussee sur la ligne de reunion de la pierre calcaire, et du granit feuillete ou gneiss pour fonder le filon dans cet endroit. Ce filon a six pouces d'epaisseur, et consiste en quartz entre-mele d'ochre martiale, de pyrite cuivreuse et galene. Cette derniere est souvent recouverte de chaux de plomb grise, et de pet.i.ts cristaux de mine de plomb jaune donnant dans l'a.n.a.lyse un indice d'or. Ce filon finit a la reunion de la pierre calcaire au gneiss. Cette reunion se fait ici dans la direction d'une heure 6/8 de la boussole de raineur, et sous un inclinaison, occidentale de 26 degres.

"Mais ce qu'il y a de remarquable, c'est que le gneis ne participe en rien de la pierre calcaire quoiqu'il n'en soit separe que par une couche d'une pouce d'epaisseur de terre argileuse et calcaire, tandis que le rocher calcaire renferme beaucoup de fragmens de granit et de gneis, dans le voisinage de cette reunion.

"Cette observation prouve incontestablement que le granit et le gneis avoient deja acquis une durete capable de resister aux infiltration des parties calcaire, et qu'ils existoient a-peu-pres tels qu'ils sont aujourd'hui lorsque la pierre calcaire commenca a se former; autrement elle n'auroit pu saisir et envelopper des morceaux detaches de ces rochers auxquels on donne avec raison l'epithete de primitif ou de premiere formation."

M. Schreiber continues his reasoning upon those mineral appearances, in adducing another argument, which I do not think equally conclusive. He says, "Le filon de la Gardette devoit pareillement exister avant la montagne calcaire, car s'il s'etoit forme apres, je ne voit pas la raison pour laquelle il s'y seroit arrete court, et pourquoi il ne se seroit pas prolonge dans cette espece de rocher." It is not necessary, in the formation of a vein, that it should proceed in traversing all the strata which then are superinc.u.mbent; it is reasonable to suppose, and consistent with observation to find them stop short in proceeding from one stratum to another. Had M. Schreiber found any pieces of the vein contained in the calcareous rock, he would have had good reason for that a.s.sertion; but, to conclude that fact from grounds which do not necessarily imply it, is not to be permitted in sound reasoning, if certainty is the object, and not mere probability.

SECT. II.--The Theory confirmed from Observations made on purpose to elucidate the subject.