THE POPULAR EDUCATOE. 



all but the most unfossiliferous of the systems, yet he made 

 them tell an interesting story, and one which, at the time, was 

 very new. 



The old red sandstone, which was so ably described, was 

 developed in Scotland ; bat the same class of rocks is found in 

 Cumberland, in Herefordshire, in Monmouthshire, and especially 

 in Devonshire. In this last locality they have been examined 

 by Mr. Londbdale, Professor Sedgwick, and Sir E. I. Murchison, 

 who considered that here the group was typically exhibited ; and 

 hence the term " Devonian " has been substituted for " Old Red 

 Sandstone," which designation was too descriptive and not 

 sufficiently inclusive, as limestones are found amongst the 

 members of the system. 



The very term " old red sandstone " would imply that there 

 was a " new red sandstone," which is the case. These two 

 similar groups are separated from each other by the carbo- 

 niferous system. Both are coloured with the red oxide of iron, 

 and in each sandstones predominate ; hence their name. 



The Devonian rocks are exceedingly difficult of classification. 

 The fossils they contain are not numerous, and moreover, as is 

 ever the case in arenaceous rocks, those which are found are 

 often very imperfect ; and as they crop out in distant localities 

 Scotland, Cumberland, Herefordshire, Devonshire, and Ireland 

 any information we might have gained from their relative 

 position is denied us. Hence there is much diversity in the 

 gaological world upon the subject of their classification. 



However, all seem to agree upon the fact that there are in 

 the British Isles two very distinct types of rock belonging to 

 the same period, occupying a position between the Silurians and 

 the carboniferous strata. These are the " old red sandstones," 

 long known to geologists, and the " Devonian " rocks. Although 

 they contain no fossils in common, and never approach each 

 other near enough to be contiguous, yet each evidently belongs 

 to the same period ; and hence these are classed together in the 

 eame system. 



On the Continent the " old red " sandstone type is not found, 

 but in the Eifel, and elsewhere, rocks appear of the same nature, 

 and containing the same fossils as the Devonian type. In 

 America there is an immense tract of country the whole state 

 of New York' occupied by rocks of this age. Here they lie 

 almost horizontal and undisturbed, and they again appear on 

 the south of the estuary of the St. Lawrence. 



Unwilling to attempt a general classification, we shall give a 

 description of the rocks in their localities, and the divisions 

 of which they are capable in each place. 



The " Old Red " of Scotland may be thus classified : 



f Yellow siliceous sandstone. 

 Uppers Impure concretionary limestone. 



(, Bed sandstone and conglomerate. 

 Middle : Grey sandstone and earthy slate. 



f'Ked and variegated sandstone. 

 Lower J Bituminous schists. 



(_Great conglomerate and red sandstone. 



The first member of the group is found at Dura Den, near 

 Cupar, in Fife, where it immediately underlies the coal. Fish 

 abound in it, such as Holoptychius, Pterichthys, Glyptopomus, 

 etc. 



It would seem that the yellow sandstones of Kilkenny are of 

 the same age as these yellow sandstones in Fife. 



The next lower rocks are found in the south of the Grampians. 

 They stretch into Fife, and when contiguous with yellow sand- 

 stones, lie beneath them ; therefore they must have been 

 deposited at an earlier period. In these rocks, near Perth, were 

 discovered the scales of a large ganoid fish, belonging to the 

 genus Holoptychius. Some of these scales were 3 inches long, 

 by 21 broad. An entire specimen of the fish has since been 

 found, and measures 2 feet in length. 



The lowermost strata afford grey paving-stones and roofing-' 

 slate ; and immediately above them lie vast accumulations of 

 conglomerate, which are characteristic of the system. 



Many geologists suppose that during the deposition of this 

 period a rise in the land took place, which would cause the 

 gravel and water- worn debris to collect in the hollows to a great 

 thickness. The position of the "old red" in the north of 

 Scotland favours this supposition. 



Hugh Miller well describes the lithological composition of 

 Scotland as a mass of granite, gneiss, and other metamorphic 

 rocks, set in a frame of sandstone ; that is, the north of Scot- 



land, from Cape Wrath to the south of the Grampians, was 

 nearly all an island in the Devonian Sea, or else the whole of 

 the Devonian deposition has been denuded at a later period. 

 This is probably the case, for here and there over the country 

 portions of Devonian rocks are found capping the high grounds. 



The grey paving-stones have afforded many remarkable fossil 

 fish. The Cephalaspis is here found. Fig. 54 exhibits the bony 

 shield which covered the head of the fish, and a coat of mail 

 which extended some way down the back. Of course the tail, 

 with its propelling fin, came out from under the extremity of its 

 armour. The peculiar shape of this fish has caused it to be 

 mistaken for a'trilobite. 



In the Arbroath paving-stones, which belong to this forma- 

 tion, the quarrymen find the fragmentary remains ot huge 

 crustaceans, which, from their wing-like form, they call " sera- 

 phim." Fig. 55 exhibits one of these fossils. It is now known 

 to be one of the anterior feet of the Ptcrygotus Anglicus 

 (pteryx, a wing, and ous, an ear). The strong spines which 

 fringe the extremity were used as masticating organs. Fig. 56 

 shows the prehensile claw of the pterygotus. This crustacean 

 must have often been six or seven feet long a lobster of no 

 mean size ! 



Another interesting feature in these beds is the frequent re- 

 currence of a number of patches of spawn-like formations. These 

 the quarrymen call " berries," a not inappropriate name, as may 

 be judged from the illustration (Fig. 57). There is every reason 

 to believe that these are the egg-packets of the crustaceans just 

 alluded to, for wherever the remains of the pterygotus occur, 

 as is frequently the case in the uppermost beds of the Silurians 

 in Wales, there these peculiar fossils are always found. They 

 have been named Parka decipiens. If the idea that they are 

 spawn or eggs of the crustaceans be correct, then the number 

 of those animals must have been enormous, if we may judge 

 from the quantity of their eggs. 



The " Old Red " in the north of Scotland differs in many of its 

 characters from that south of the Grampians. Miller supposed 

 that it was the bottom of the "old red group," because at 

 Cromarty he found the sandstone beds reclining on beds more 

 or less crystalline, which he considered were the floor of the 

 system. This, however, further search has proved is not the 

 case, for in 1861 remains of the pterygotus were found many 

 hundred feet below this North Scotland "old red," which of 

 course proves that it was a later deposit than that south of the 

 Grampians, of which the pterygotus is characteristic. 



Leaving the North, we now turn to Devonshire, where the 

 system is typically developed. The following division is that 

 given by Murchison and Sedgwick : 



1. The Upper or Pilton Group. 



2. The Middle or Ilfraconbe Group. 



3. The Lower or Linton Group. 



The Lower Devonian consists of soft slates, mixed with sand- 

 stones, which are found near Lynmouth, and the celebrated 

 " valley of rocks " exhibits this group. The trilobites, linger- 

 ing beyond the Silurian era, are found in this strata, together 

 with corals, orthis, and spirifers. 



Another member of the Lower Devonian is shown a few miles 

 to the east, at Linton. Here are hard, reddish, and purple sand- 

 stones, containing but few fossils. 



The Middle Devonian, containing the limestone of the system, 

 is the most productive of fossils. This limestone is exhibited 

 at Plymouth and Torbay, and its equivalent is found at Ilfra- 

 combe and Combe Martin. These rocks are replete with fossils, 

 especially corals, such as Favosites, Heliolites, and Cyathophyl- 

 lum, some of which lived through the Silurian, the Devonian, 

 and Carboniferous periods. The most common of the fossil 

 shells are : Stringocephalus (Fig. 58a ; and side view, 58&) ; 

 Megalodon cucullatus (Fig. 59) ; Calceola (Fig. 60) ; Spirifer 

 (Fig. 61) ; Murchisonia (Fig. 62) ; Pleurotomaria (Fig. 63). 



The prominent member of the Devonian on the Continent, the 

 " Eifel limestone," is equivalent to our Middle Devonian. 



The Upper Devonian consists of brown and yellow sandstones, 

 which begin to give evidence of the vegetable life which reached 

 its culminating point in the next system the carboniferous. 



Above these sandstones are calcareous brown slates, which 

 appear in the neighbourhood of Pilton and Barnstaple. Their 

 fossils show an approach to the carboniferous life, yet they are 

 sufficiently distinct to prevent these strata from bein? classed as 

 carboniferous. 



