1871 .] KAMSAY PRETRIASSIC EED BOCKS. 243 



of the Old Red strata, in which there are no Mollusca, are species of 

 Auchenasj)is, Onchus (2 species), Pteraspis, Cephalaspis, and Plec- 

 trodtis. These also in the main indicate a change of conditions, 

 which were, I believe, of a geographical kind. 



The Eurypteridse and Pterygoti in England almost entirely belong 

 to the passage-beds ; and one, Eurypterus Symondsii, is only found 

 in the lower Old Eed strata. 



The circumstances which marked the passage of the uppermost 

 Silurian rocks into Old Red Sandstone seem to me to have been the 

 following : — First, a shallowing of the sea, followed by a gradual 

 alteration in the physical geography of the district, so that the area 

 became changed into a series of mingled fresh and brackish lagoons, 

 which finally, by continued terrestrial changes were converted into 

 a great freshwater lake, or, if we take the whole of Britain and lands 

 beyond, into a series of lakes ; and the occurrence of a very few 

 genera or even species of fish and Crustacea, common both to the 

 fresh and the brackish or even salt waters, does not prove that the 

 Old Red Sandstone is truly marine. At the present day animals 

 that are commonly supposed to be essentially marine, are occa- 

 sionally found inhabiting fresh water. Thus I am informed by 

 Mr. Murray, of the Geological Survey of Canada, that in the inland 

 fresh lakes of Newfoundland seals are common. They breed there 

 freely, and never visit the sea. The same is the case in Lake Baikal, 

 in Central Asia ; and though these facts bear but slightly on my pre- 

 sent subject, seals being air-breathing MammaHa, yet in some of 

 the lakes of Sweden it is said that marine Crustacea are found. 

 This may be accounted for in the same way that I now attempt to 

 account for these peculiarities in the Old Red Sandstone strata. 

 These Swedish lakes were submerged during the Glacial period ; 

 and being deep basins (scooped out it matters not by what pro- 

 cess), while the land was emerging, and after its final emergence, 

 the salt water of the lakes freshened so slowly, that some of the 

 creatures inhabiting it had time by degrees to adjust themselves to 

 new and abnormal conditions*. 



Again, we may suppose a set of circumstances such as the follow- 

 ing : — If by changes of physical geography of a continental kind 

 a portion of the Silurian sea got isolated from the main ocean, more 

 or less like the Caspian and the Black Sea, then the ordinary marine 

 conditions of the " passage beds," accompanied by some of the life 

 of the period, might be maintained for what, in common language, 

 seems to us a long time. The Black Sea was once united to the 

 Caspian, the two together forming one great brackish lake. The 

 Black Sea is now steadily freshening ; and it is easy to conceive that 

 by the closing of the Bosphorus (a comparatively small geographical 

 change) it might be again converted into a fresh lake. At present 

 a great body of salt water is constantly being poured out through the 

 Bosphorus, and its place taken by the fresh water of rivers. At pre- 



* For much important information on this subject, see Annals and Mag. of 

 Nat. Hist., 3rd series, vol. i. 1858, p. 60, " On the Occurrence of Marine Animal 

 Forms in Fresh Water," by Dr. B. von Martens. Translated by Mr. W. S. Dallas. 



