Principles of Geology Part 66

CHAPTER XLIX.

IMBEDDING OF AQUATIC SPECIES IN SUBAQUEOUS STRATA.

Inhumation of fresh water plants and animals--Sh.e.l.l marl--Fossilized seed-vessels and stems of chara--Recent deposits in American lakes--Freshwater species drifted into seas and estuaries--Lewes levels--Alternations of marine and freshwater strata, how caused--Imbedding of marine plants and animals--Cetacea stranded on our sh.o.r.es--Littoral and estuary Testacea swept into the deep sea--Burrowing sh.e.l.ls--Living Testacea found at considerable depths--Blending of organic remains of different ages.

Having treated of the imbedding of terrestrial plants and animals, and of human remains, in deposits now forming beneath the waters, I come next to consider in what manner _aquatic_ species may be entombed in strata formed in their own element.

_Freshwater plants and animals._--The remains of species belonging to those genera of the animal and vegetable kingdoms which are more or less exclusively confined to fresh water are for the most part preserved in the beds of lakes or estuaries, but they are oftentimes swept down by rivers into the sea, and there intermingled with the exuviae of marine races. The phenomena attending their inhumation in lacustrine deposits are sometimes revealed to our observation by the drainage of small lakes, such as are those in Scotland, which have been laid dry for the sake of obtaining sh.e.l.l marl for agricultural uses.

In these recent formations, as seen in Forfarshire, two or three beds of calcareous marl are sometimes observed separated from each other by layers of drift peat, sand, or fissile clay. The marl often consists almost entirely of an aggregate of sh.e.l.ls of the genera Limnea, Planorbis, Valvata, and Cyclas, of species now existing in Scotland. A considerable proportion of the Testacea appear to have died very young, and few of the sh.e.l.ls are of a size which indicates their having attained a state of maturity. The sh.e.l.ls are sometimes entirely decomposed, forming a pulverulent marl; sometimes in a state of good preservation. They are frequently intermixed with stems of Charae and other aquatic vegetables, the whole being matted together and compressed, forming laminae often as thin as paper.

_Fossilized seed-vessels and stems of Chara._--As the Chara is an aquatic plant which occurs frequently fossil in formations of different eras, and is often of much importance to the geologist in characterizing entire groups of strata, I shall describe the manner in which I have found the recent species in a petrified state. They occur in a marl-lake in Forfarshire, inclosed in nodules, and sometimes in a continuous stratum of a kind of travertin.

[Ill.u.s.tration: Fig. 102.

Seed-vessel of Chara hispida.

_a_, Part of the stem with the seed-vessel attached. Magnified.

_b_, Natural size of the seed vessel.

_c_, Integument of the Gyrogonite, or petrified seed-vessel of _Chara hispida_, found in the Scotch marl-lakes.

Magnified.

_d_, Section showing the nut within the integument.

_e_, Lower end of the integument to which the stem was attached.

_f_, Upper end of the integument to which the stigmata were attached.

_g_, One of the spiral valves of _c_.

The seed-vessel of these plants is remarkably tough and hard, and consists of a membranous nut covered by an integument (_d_, fig. 102.) both of which are spirally striated or ribbed. The integument is composed of five spiral valves, of a quadrangular form (_g_). In _Chara hispida_, which abounds in the lakes of Forfarshire, and which has become fossil in the Bakie Loch, each of the spiral valves of the seed-vessel turns rather more than twice round the circ.u.mference, the whole together making between ten and eleven rings. The number of these rings differs greatly in different species, but in the same appears to be very constant.

The stems of Charae occur fossil in the Scotch marl in great abundance.

In some species, as in _Chara hispida_, the plant when living contains so much carbonate of lime in its vegetable organization, independently of calcareous incrustation, that it effervesces strongly with acids when dry. The stems of _Chara hispida_ are longitudinally striated, with a tendency to be spiral. These striae, as appears to be the case with all Charae, turn always like the worm of a screw from right to left, while those of the seed-vessel wind round in a contrary direction. A cross section of the stem exhibits a curious structure, for it is composed of a large tube surrounded by smaller tubes (fig. 103., _b_, _c_) as is seen in some extinct as well as recent species. In the stems of several species, however, there is only a single tube.[1099]

[Ill.u.s.tration: Fig. 103.

Stem and branches of Chara hispida.

_a_, Stem and branches of the natural size.

_b_, Section of the stem magnified.

_c_, Showing the central tube surrounded by two rings of smaller tubes.

The valves of a small animal called cypris (_C. ornata?_ Lam.) occur completely fossilized, like the stems of Charae, in the Scotch travertin above mentioned. The same cypris inhabits the lakes and ponds of England, where, together with many other species, it is not uncommon.

Although extremely minute, they are visible to the naked eye, and may be observed in great numbers, swimming swiftly through the waters of our stagnant pools and ditches. The antennae, at the end of which are fine pencils of hair, are the princ.i.p.al organs for swimming, and are moved with great rapidity. The animal resides within two small valves, not unlike those of a bivalve sh.e.l.l, and moults its integuments annually, which the conchiferous mollusks do not. The cast-off sh.e.l.ls, resembling thin scales, and occurring in countless myriads in many ancient freshwater marls, impart to them a divisional structure, like that so frequently derived from plates of mica.

[Ill.u.s.tration: Fig. 104.

_Cypris unifasciata_, a living species, greatly magnified.

_a_, Upper part. _b_, Side view of the same.

[Ill.u.s.tration: Fig. 105.]

_Cypris vidua_, a living species, greatly magnified.[1100]]

The recent strata of lacustrine origin above alluded to are of very small extent, but a.n.a.logous deposits on the grandest scale are forming in the great Canadian lakes, as in Lakes Superior and Huron, where beds of sand and clay are seen inclosing sh.e.l.ls of existing species.[1101]

The Chara also plays the same part in the subaqueous vegetation of North America as in Europe. I observed along the borders of several freshwater lakes in the state of New York a luxuriant crop of this plant in clear water of moderate depth, rendering the bottom as verdant as a gra.s.sy meadow. Here, therefore, we may expect some of the tough seed vessels to be preserved in mud, just as we detect them fossil in the Eocene strata of Hampshire, or in the neighborhood of Paris, and many other countries.

_Imbedding of freshwater Species in Estuary and Marine Deposits._

_In Lewes levels._--We have sometimes an opportunity of examining the deposits which within the historical period have silted up some of our estuaries; and excavations made for wells and other purposes, where the sea has been finally excluded, enable us to observe the state of the organic remains in these tracts. The valley of the Ouze between Newhaven and Lewes is one of several estuaries from which the sea has retired within the last seven or eight centuries; and here, as appears from the researches of Dr. Mantell, strata thirty feet and upwards in thickness have acc.u.mulated. At the top, beneath the vegetable soil, is a bed of peat about five feet thick, inclosing many trunks of trees. Next below is a stratum of blue clay containing freshwater sh.e.l.ls of about nine species, such as now inhabit the district. Intermixed with these was observed the skeleton of a deer. Lower down, the layers of blue clay contain, with the above-mentioned freshwater sh.e.l.ls, several marine species well known on our coast. In the lowest beds, often at the depth of thirty-six feet; these marine Testacea occur without the slightest intermixture of fluviatile species, and amongst them the skull of the narwal, or sea unicorn (_Monodon monoceros_), has been detected.

Underneath all these deposits is a bed of pipe-clay, derived from the subjacent chalk.[1102]

If we had no historical information respecting the former existence of an inlet of the sea in this valley and of its gradual obliteration, the inspection of the section above described would show, as clearly as a written chronicle, the following sequence of events. First, there was a salt-water estuary peopled for many years by species of marine Testacea identical with those now living, and into which some of the larger Cetacea occasionally entered. Secondly, the inlet grew shallower, and the water became brackish, or alternately salt and fresh, so that the remains of freshwater and marine sh.e.l.ls were mingled in the blue argillaceous sediment of its bottom. Thirdly, the shoaling continued until the river-water prevailed, so that it was no longer habitable by marine Testacea, but fitted only for the abode of fluviatile species and aquatic insects. Fourthly, a peaty swamp or mora.s.s was formed, where some trees grew, or perhaps were drifted during floods, and where terrestrial quadrupeds were mired. Finally, the soil being flooded by the river only at distant intervals, became a verdant meadow.

_In delta of Ganges and Indus._--It was before stated, that on the sea-coast, in the delta of the Ganges, there are eight great openings, each of which has evidently, at some ancient period, served in its turn as the princ.i.p.al channel of discharge.[1103] As the base of the delta is 200 miles in length, it must happen that, as often as the great volume of river-water is thrown into the sea by a new mouth, the sea will at one point be converted from salt to fresh, and at another from fresh to salt; for, with the exception of those parts where the princ.i.p.al discharge takes place, the salt water not only washes the base of the delta, but enters far into every creek and lagoon. It is evident, then, that repeated alternations of beds containing freshwater sh.e.l.ls, with others filled with marine exuviae, may here be formed. It has also been shown by artesian borings at Calcutta (see above, p. 267), that the delta once extended much farther than now into the gulf, and that the river is only recovering from the sea the ground which had been lost by subsidence at some former period. a.n.a.logous phenomena must sometimes be occasioned by such alternate elevation and depression as has occurred in modern times in the delta of the Indus.[1104] But the subterranean movements affect but a small number of the deltas formed at one period on the globe; whereas the silting up of some of the arms of great rivers and the opening of others, and the consequent variation of the points where the chief volume of their waters is discharged into the sea, are phenomena common to almost every delta.

The variety of species of Testacea contained in the recent calcareous marl of Scotland, before mentioned, is very small, but the abundance of individuals extremely great, a circ.u.mstance very characteristic of freshwater formations in general, as compared to marine; for in the latter, as is seen on sea-beaches, coral-reefs, or in the bottom of the seas examined by dredging, wherever the individual sh.e.l.ls are exceedingly numerous, there rarely fails to be a vast variety of species.

_Imbedding of the Remains of Marine Plants and Animals._

_Marine plants._--The large banks of drift sea-weed which occur on each side of the equator in the Atlantic, Pacific, and Indian oceans, were before alluded to.[1105] These, when they subside, may often produce considerable beds of vegetable matter. In Holland, submarine peat is derived from Fuci, and on parts of our own coast from _Zostera marina_.

In places where Algae do not generate peat, they may nevertheless leave traces of their form imprinted on argillaceous and calcareous mud, as they are usually very tough in their texture.

Sea-weeds are often cast up in such abundance on our sh.o.r.es during heavy gales, that we cannot doubt that occasionally vast numbers of them are imbedded in littoral deposits now in progress. We learn from the researches of Dr. Forchhammer, that besides supplying in common with land plants the materials of coal, the Algae must give rise to important chemical changes in the composition of strata in which they are imbedded. These plants always contain sulphuric acid, and sometimes in as large a quant.i.ty as 8 per cent., combined with potash: magnesia also and phosphoric acid are constant ingredients. Whenever large ma.s.ses of sea-weeds putrefy in contact with ferruginous clay, sulphuret of iron, or iron pyrites, is formed by the union of the sulphur of the plants with the iron of the clay; while the potash, released from its union with the clay (_i. e._ silicate of alumina), forms with it a peculiar compound. Many of the mineral characteristics of ancient rocks, especially the alum slates, and the pyrites which occur in clay slate, and the fragments of anthracite in marine Silurian strata, may be explained by the decomposition of fucoids or sea-weeds.[1106]

_Imbedding of cetacea._--It is not uncommon for the larger Cetacea, which can float only in a considerable depth of water, to be carried during storms or high tides into estuaries, or upon low sh.o.r.es, where, upon the retiring of high water, they are stranded. Thus a narwal (_Monodon monoceros_) was found on the beach, near Boston in Lincolnshire, in the year 1800, the whole of its body buried in the mud.

A fisherman going to his boat saw the horn, and tried to pull it out, when the animal began to stir itself.[1107] An individual of the common whale (_Balaena mysticetus_), which measured seventy feet, came ash.o.r.e near Peterhead, in 1682. Many individuals of the genus Balaenoptera have met the same fate. It will be sufficient to refer to those cast on sh.o.r.e near Burnt Island, and at Alloa, recorded by Sibbald and Neill. The other individual mentioned by Sibbald, as having come ash.o.r.e at Boyne, in Banffshire, was probably a razor-back. Of the genus Catodon (_Cachalot_), Ray mentions a large one stranded on the west coast of Holland in 1598, and the fact is also commemorated in a Dutch engraving of the time of much merit. Sibbald, too, records that a herd of Cachalots, upwards of 100 in number, were found stranded at Cairston, in Orkney. The dead bodies of the larger Cetacea are sometimes found floating on the surface of the waters, as was the case with the immense whale exhibited in London in 1831. And the carcase of a sea-cow or Lamantine (_Halicora_) was, in 1785, cast ash.o.r.e near Leith.