218 P. Macnair Sf J. Reid — Palceontological Considerations — 



globe, is by no means a proof of tbe fresb-water origin of tbe latter 

 fishes. Tbe immense interval of time implied by tbe comparison 

 renders its value somewbat problematical. Sir A. Geikie, in re- 

 ferring to tbe fisbes of tbe Old Red Sandstone, says : " Tbat some 

 of tbe fisbes found tbeir way to tbe sea, as our modern salmon does, 

 is indicated by the occasional occurrence of tbeir remains among 

 tbose of tbe truly marine fauna of tbe Devonian rocks." ' 



Tbe life bistory, however, of the salmon presents evidence of 

 a somewhat different kind ; for we find tbat the Salmonidaa made 

 their appearance for the first time in the Chalk of tbe Cretaceous 

 system, 2 which would rather indicate a migration from tbe sea. In 

 like manner, the life history of these obscure representatives of tbe 

 ganoids of the Old Bed Sandstone would probably show that they 

 preserved their existence " in tbe struggle for life " by a successful 

 adaptation to new surroundings — namely, fresh-water conditions — 

 while their less-favoured contemporaries succumbed. 



The large numbers of fish-remains entombed in the Caithness 

 flags indicate a widespread and sudden destruction of life. Earth- 

 quake shocks, the discharge of mephitic gases, and heated waters 

 have been respectively suggested as the cause of the catastrophe. 

 But whatever the agent of destruction may have been, it apparently 

 came in fi'om an outside area, sweeping before it the fishes of the 

 Old Eed Sandstone of Caithness inward towards the shallow shores, 

 where they perished. In referring to the flagstone quarry of 

 Acbanarras and its contained fish-remains, Dr. B. H. Traquair states 

 that in the lower bed, two and a half feet in thickness, " there all 

 the species in the list occur almost indiscriminately." 3 It is im- 

 possible to conceive the manner in which dead fisbes could be 

 preserved entire, during tbe formation of sandstones and shales of 

 several feet in thickness, in a tideless fresh-water lake. But a 

 shallow seashore, presenting broad stretches of sandbanks and 

 mud-flats, would afford a more adequate medium for this purpose. 

 Tbe diurnal tides, on tbe one hand, laden with sand and mud in 

 tbeir ebb and flow, would undoubtedly cover up immense numbers 

 of the fisbes in question ; while, on the other, tbe antiseptic properties 

 of salt water would tend to their preservation, even for some time 

 afterwards. 4 



The Introduction and Geographical Distribution of the Old Red 

 Sandstone Fishes. 

 With regard to the introduction and geographical distribution of 

 tbe fishes of the Old Bed Sandstone, the case of the " Lake theory " 

 displays conspicuous weakness. The Eui-ypterids and Cephalaspidians 

 of the Upper Silurian are in evidence ; but of the genera Osteolepis, 

 Dipterus, Glyptolepis, and other fishes of the Old Bed Sandstone, 

 no undoubted plates or scales occur in the preceding formation. 

 Tbe question therefore arises, whence came these highly organized 



1 "Classbook of Geology," 1890, p. 262. 



2 Giinther, " Study of Fishes," 1880, p. 631. 

 5 Proc. Rot. Phys. Soc. Edinburgh, vol. xii, p. 283. 

 4 Proc. Perthshire Soc. Nat. Sci. 1893-4. 



