J. Lomas — The Crystalline Gneisses. 541 



The characters of the oldest sedimentary rocks lend aid to the 

 theory that they were laid down in long, parallel trough-like 

 depressions. Dr. Hicks, speaking of the Lower Cambrian, says : ^ 

 " The deposits must have accumulated with comparative rapidity 

 in fairly shallow water." Shallow-water deposits imply proximity 

 to land. We cannot obtain shallow-water rocks, extending as they 

 do in separate troughs from Britain to Russia, on the assumption 

 that they were laid down in a continuous sea extending across 

 Europe. Dr. Hicks many years ago propounded this theory to 

 account for the distribution of the Cambrian rocks.^ 



It is probable that the primitive ridges were not very lofty. They 

 would, no doubt, form the counterparts of the troughs. Professor 

 Lapworth has shown ^ how persistently a lofty fold is counter- 

 balanced by a deep parallel depression, and a low ridge by a shallow 

 trough. Dr. Hicks suggests * that high mountains existed in 

 Pre-Cambrian times, and that some of them were probably snow- 

 covered. So far as I can gather, the only reason why he postulates 

 these high mountains is to explain the occurrence in the Cambrian 

 conglomerates of boulders over a foot in diameter. In mountain- 

 building, as in everything else about us, there has been a gradual 

 evolution going on. The Tertiary is the epoch of mountain-building, 

 and there has probably been a progression in the heights of 

 mountains from ancient to modern times. 



The tracing out of old shorelines, and the mapping of former 

 distribution of land and water, form a branch of geology which has 

 of late come into prominence. 



Pioneer work has been done by Godwin-Austen, Professor Hull,^ 

 Jukes-Browne,'' and others, but much will need to be done before 

 we can draw maps which will show the distribution of land and 

 water in Paleeozoic times with accui-acy. The maps published by 

 Professor Hull and Jukes- Browne must not be taken as being 

 absolutely correct, especially in the Palteozoic period. We may, 

 however, regard them as approximations to the truth. I had 

 occasion recently to copy out on tracing-paper the Paleozoic shore- 

 lines as given by Jukes-Browne. On accurately registering these 

 and holding them up to the light, I was struck with the fact that in 

 some districts there was a great closing up of shorelines. This was 

 especially noticeable about Anglesey, where within a few miles of 

 each other the Cambrian, Ordovician, Silurian, Carboniferous, and 

 Permian shorelines are found. It would seem as though in 

 Anglesey we were dealing with a very stable part of the earth's 

 crust, a part which had kept its position relatively to the sea 

 nearly the same through long ages. As there is every reason to 



1 President's Address, Geological Society, 1897. 



2 Q.J.G.S. 1875, p. 552. 



3 President's Address, Section C, Britisli Association, 1892. 

 * Geological Magazine, 1876, p. 157. 



s " Contributions to the Physical History of the British Isles." 

 « " The Buildino- of the British Isles." 



