366 J. BARRELL FLUVIATILE ORIGIN OF OLD RED SANDSTONE 



fault zone. The thickness is given as nearly 3,000 feet, and from a thick 

 helt of gray and greenish mudstones and shales fossils have been ob- 

 tained — phyllocarids, myriopods, eurypterids, fragments of scorpions, 

 plant fragments, and worm tracks. Further, a thin bed of reddish mud- 

 stone underlying the above series has yielded numerous plates of a new 

 Cyathaspis. 23 Of these fossils the phyllocarids, as an order, are typically 

 marine; the myriopods and scorpions are as typically terrestrial. The 

 phyllocarids belong, however, to the class of crustaceans which has always 

 exhibited a freedom of adaptation to changes of salinity, possibly greater 

 than is found in any other group of organisms. 



From these descriptions of uppermost Silurian rocks and their fossils, 

 judged in the light of more intensive studies of other delta deposits, it 

 would seem that toward the close of this period delta conditions had 

 developed from the northwest, pushing the littoral zone south of Scotland. 

 The Scottish deposits were dominantly subaerial, forming in the main 

 fluviatile delta deposits in Downtonian times. Occasionally, as is found 

 on the fronts of all large deltas of low gradient, submergence would bring 

 in wide shallow bays of brackish waters with such groups of animals as 

 were least sensitive to changes of salinity. At rare intervals this faun a I 

 invasion reached as far inland as southern or central Scotland. The 

 climate was dry enough to permit drying out and oxidation of the deposits 

 of the floodplains, but there is nothing in the chemical compositions or 

 sedimentary structures to imply the existence of a desert climate. 



The evidence is not so determinative as could be wished, partly no 

 doubt because it has not been searched bed by bed for the structural 

 features, partly because evidence which may in another decade be re- 

 garded as determinative is not generally regarded as such at the present 

 time. 



LOWER DEVONIAN— LOWER OLD RED 



In Cornwall and southern Devon all, or nearly all, of the Lower, as in 

 fact the rest here of the Devonian also, is marine. In southern Ireland, 

 however, the beds are largely or entirely fresh water in origin, since no 

 marine fossils have ever been found there. 



In south Wales the Lower Old Eed is represented by the cornstone 

 series. This consists below of red marls with bands of nodular limestones 

 (cornstones) and irregular beds of red micaceous sandstones. Above are 

 the Senni beds, consisting of green and dull red sandstones with marls 

 and cornstone conglomerates. 



23 The Downtonian and Old Red Sandstone of Kincardineshire. Geol. Mag., Decade 5, 

 vol. ix, 1912, p. 511. 



