A Manual of Elementary Geology Part 51

[Ill.u.s.tration: Fig. 388. _Cyathocrinites pla.n.u.s_, Miller.

Mountain limestone.]

[Ill.u.s.tration: Fig. 389. _Spirifer glaber_, Sow. Mountain limestone.]

[Ill.u.s.tration: Fig. 390. _Productus Martini_, Sow. (_P. semireticulatus_, Flem.) Mountain limestone.]

Among the spiral univalve sh.e.l.ls the extinct genus _Euomphalus_ (see fig.

391.) is one of the commonest fossils of the Mountain limestone. In the interior it is often divided into chambers (see fig. 391. _d_); the septa or part.i.tions not being perforated, as in foraminiferous sh.e.l.ls, or in those having siphuncles, like the Nautilus. The animal appears, like the recent _Bulimus decollatus_, to have retreated at different periods of its growth, from the internal cavity previously formed, and to have closed all communication with it by a septum. The number of chambers is irregular, and they are generally wanting in the innermost whorl.

[Ill.u.s.tration: Fig. 391. _Euomphalus pentagulatus_, Min. Con.

Mountain limestone.

_a._ Upper side; _b._ lower, or umbilical side; _c._ view showing mouth which is less pentagonal in older individuals; _d._ view of polished section, showing internal chambers.]

[Ill.u.s.tration: Fig. 392. Portion of _Orthoceras laterale_, Phillips. Mountain limestone.]

There are also many univalve and bivalve sh.e.l.ls of existing genera in the Mountain limestone, such as _Turritella_, _Buccinum_, _Patella_, _Isocardia_, _Nucula_, and _Pecten_.[341-A] But the _Cephalopoda_ depart, in general, more widely from living forms, some being generically distinct from all those found in strata newer than the coal. In this number may be mentioned _Orthoceras_, a siphuncled and chambered sh.e.l.l, like a _Nautilus_ uncoiled and straightened. Some species of this genus are several feet long (fig. 392.). The _Goniat.i.te_ is another genus, nearly allied to the _Ammonite_, from which it differs in having the lobes of the septa free from lateral denticulations, or crenatures; so that the outline of these is continuous and uninterrupted (see _a_, fig. 393.). Their siphon is small, and in the form of the striae of growth they resemble _Nautili_. Another extinct generic form of Cephalopod, abounding in the Mountain limestone, and not found in strata of later date, is the _Bellerophon_ (fig. 394.), of which the sh.e.l.l, like the living Argonaut, was without chambers.

[Ill.u.s.tration: Fig. 393. _Goniat.i.tes evolutus_, Phillips.[342-A]

Mountain limestone.]

[Ill.u.s.tration: Fig. 394. _Bellerophon costatus_, Sow.[342-B]

Mountain limestone.]

FOOTNOTES:

[329-A] H. D. Rogers, Trans. a.s.soc. Amer. Geol., 1840-42, p. 440.

[333-A] Trans. of a.s.s. of Amer. Geol., p. 470.

[334-A] Lyell's Second Visit to the U. S., vol. ii. p. 245. American Journ. of Sci., 2d series, vol. v. p. 17.

[335-A] Principles of Geol., p. 696.

[335-B] For changes in climate, see Principles of Geol., chaps.

vii. and viii.

[335-C] Geol. Trans., 2d series, vol. vi. p. 330.

[336-A] Aga.s.siz, Poiss. Foss., lib. 4. p. 62. and liv. 5. p. 88.

[337-A] Goldfuss, Neue Jenaische Lit. Zeit., 1848; and Von Meyer, Quart.

Geol. Journ., vol. iv. p. 51., memoirs.

[338-A] See Lyell's Second Visit, &c., vol. ii. p. 305.

[340-A] These impressions, found by Mr. Lea, were imagined to be in a rock as ancient as the old red sandstone; but, according to Mr. H. D. Rogers, they are in the lowest part of the coal formation.

[341-A] Phillips, Geol. of Yorksh., vol. ii. p. 208.

[342-A] Phillips, Geol. of Yorksh., pl. 20. fig. 65.

[342-B] Ibid., pl. 17. fig. 15.

CHAPTER XXVI.

OLD RED SANDSTONE, OR DEVONIAN GROUP.

Old Red Sandstone of Scotland, and borders of Wales--Fossils usually rare--"Old Red" in Forfarshire--Ichthyolites of Caithness--Distinct lithological type of Old Red in Devon and Cornwall--Term "Devonian"--Organic remains of intermediate character between those of the Carboniferous and Silurian systems--Corals and sh.e.l.ls--Devonian strata of Westphalia, the Eifel, Russia, and the United States--Coral reef at Falls of the Ohio--Devonian flora.

It was stated in Chap. XXII. that the Carboniferous formation is surmounted by one called the "New Red," and underlaid by another called the "Old Red Sandstone."[342-C] The British strata of the last mentioned series were first recognized in Herefordshire and Scotland as of great thickness, and immediately subjacent to the coal; but they were in general so barren of organic remains, that it was difficult to find paleontological characters of sufficient importance to distinguish them as an independent group. In Scotland, and on the borders of Wales, the "Old Red" consists chiefly of red sandstone, conglomerate, and shale, with few fossils; but limestones of the same age, peculiarly rich in organic remains, were at length found in Devonshire.

I shall first advert to the characters of the group as developed in Herefordshire, Worcestershire, Shropshire, and South Wales. Its thickness has been estimated at 8000 feet, and it has been subdivided into--

1st. A quartzose conglomerate pa.s.sing downwards into chocolate-red and green sandstone and marl.

2d. Cornstone and marl--red and green argillaceous spotted marls, with irregular courses of impure concretionary limestone, provincially called Cornstone.

Here, as usual, fossils are extremely rare in the clays and sandstones in which the red oxide of iron prevails; but remains of fishes of the genera _Cephalaspis_ and _Onchus_ have been discovered in the Cornstone.

The whole of the northern part of Scotland, from Cape Wrath to the southern flank of the Grampians, has been well described by Mr. Miller as consisting of a nucleus of granite, gneiss, and other hypogene rocks, which seem as if set in a sandstone frame.[343-A] The beds of the Old Red Sandstone const.i.tuting this frame, may once perhaps have extended continuously over the entire Grampians before the upheaval of that mountain range; for one band of the sandstone follows the course of the Moray Frith far into the interior of the great Caledonian valley; and detached hills and island-like patches occur in several parts of the interior, capping some of the higher summits in Sutherlandshire, and appearing in Morayshire like oases among the granite rocks of Strathspey. On the western coast of Ross-shire, the Old Red forms those three immense insulated hills before described (p. 67.), where beds of horizontal sandstone, 3000 feet high, rest unconformably on a base of gneiss, attesting the vast denudation which has taken place.

But in order to observe the uppermost part of the Old Red, we must travel south of the Grampians, and examine its junction with the bottom of the Carboniferous series in Fifeshire. This upper member may be seen in Dura Den, south of Cupar, to consist of a belt of yellow sandstone, in which Dr.

Fleming first discovered scales of _Holoptychius_, and in which species of fish of the genera _Pterichthys_, _Pamphractus_, and others, have been met with. (For genus _Pterichthys_, see fig. 400. p. 345.)

The beds next below the yellow sandstone are well seen in the large zone of Old Red which skirts the southern flank of the Grampians from Stonehaven to the Frith of Clyde. It there forms, together with trap, the Sidlaw Hills and the strata of the valley of Strathmore. A section of this region has been already given (p. 48.), extending from the foot of the Grampians in Forfarshire to the sea at Arbroath, a distance of about 20 miles, where the entire series of strata is several thousand feet thick, and may be divided into three princ.i.p.al ma.s.ses: 1st, and uppermost, red and mottled marls, cornstone, and sandstone (Nos. 1. and 2. of the section); 2d, Conglomerate, often of vast thickness (No. 3. ibid.); 3d, Roofing and paving stone, highly micaceous, and containing a slight admixture of carbonate of lime (No. 4. ibid.). In the first of these divisions, which may be considered as succeeding the yellow sandstone of Fifeshire before mentioned, a gigantic species of fish of the genus _Holoptychius_ has been found at Clashbinnie near Perth. Some scales (see fig. 395.) have been seen which measured 3 inches in length by 2-1/2 in breadth.

At the top of the next division, or immediately under the conglomerate (No. 3. p. 48.), there have been found in Forfarshire some remarkable crustaceans, with several fish of the genus named by Aga.s.siz _Cephalaspis_, or "buckler-headed," from the extraordinary shield which covers the head (see fig. 396.), and which has often been mistaken for that of a trilobite, of the division _Asaphus_.

[Ill.u.s.tration: Fig. 395. Scale of _Holoptychius n.o.bilissimus_, Agas.

Clashbinnie. Nat. size.]

Species of the same genus are considered in England as characteristic of the second or Cornstone division (p. 343.).

[Ill.u.s.tration: Fig. 396. _Cephalaspis Lyellii_, Aga.s.s. Length 6-3/4 inches.

From a specimen in my collection found at Glammiss, in Forfarshire. See other figures, Aga.s.siz, vol. ii. tab. 1. _a_. and 1. _b_.

_a._ One of the peculiar scales with which the head is covered when perfect. These scales are generally removed, as in the specimen above figured.

_b, c._ Scales from different parts of the body and tail.]

[Ill.u.s.tration: Fig. 397. _Eggs of gasteropodous mollusk?_ Lower beds of Old Red, Ley's Mill, Forfarshire.]

[Ill.u.s.tration: Fig. 398. _Fucoids and eggs of gasteropodous mollusk?_ Lower Old Red, Fife.]