34 PALEOZOIC TIME. [CHAP. III. 



4. A great limestone series, believed to be over 1,400 feet thick. 



3. Calcareous grits and mudstones, about 80 feet. 



2. Fine-grained quartzites, with worm casts, 300 feet. 



1. Coarse quartzites, with a thin conglomerate at the base, 200 feet. 



Many fossils occur in the limestones, and so far as they 

 are yet known it seems probable that the beds are of the 

 same age as the Orthoceras limestone of Sweden and the 

 Trenton limestone of Canada, viz., Upper Arenig or Lower 

 Llandeilo. 1 If this is so, the basement beds can hardly be 

 older than the Arenig of Wales. No break or overlap 

 appears to exist in the series, but the group as a whole 

 rests un conformably on the older rocks, lying partly on 

 the Torridon sandstone, and partly on the Archaean gneiss. 



The succession above mentioned plainly indicates the 

 gradual subsidence of a land surface and its submergence 

 beneath an increasing depth of water. Mr. B. N. Peach 

 draws the following inferences from the characters of the 

 rock : 3 "In the case of the basal quartzites, where we 

 have a passage from a land surface to a sea-bed, there is 

 little or no organic matter mixed with the coarse siliceous 

 sand, which from its coarse texture, and the false-bedding 

 of the layers, bears evidence of rapid accumulation. There 

 would therefore be no food for the support of Ann elides 

 under these conditions. But with the slower accumulation 

 of sediment indicated by the 'pipe-rock,' there was evi- 

 dently time for the fertilization of the sand by the shower 

 of minute pelagic organisms which is ever falling on the 

 sea-floor, so that it could afford food for the burrowing 

 annelids, whose casts now form the stony 'pipes.' . . . 

 As the sea-floor gradually subsided, the shore-line was re- 

 moved farther from the area of deposit, and hence during 

 the deposition of the 'fucoid beds' only the finest sedi- 



1 See J. W. Salter, " Quart. Journ. Geol. Soc.," vol. xv. p. 381, and 

 J. E. Marr, "Classif. of Cambrian and Silurian Rocks.," 1883, Cam- 

 bridge, p. 67. 



2 " Proc. Roy. Phys. Soc. of Edinburgh, 1886," vol. ix. pt. i. p. 4. 



