536 Mr. Archibald GeiMe [June 7, 



begin to be worn down. That erosion took jilace during a pro- 

 longed period, and to a vast extent, is shown by the fact that in some 

 places the thick cake of sandstone was hollowed out down to the 

 Archsean platform below it before the next succeeding formations 

 were deposited. Here again we are presented with a striking example 

 of the imperfection of the Geological Kecord. 



III. The Silurian Period. 



After the long interval of time represented by the elevation of the 

 red sandstones into dry land, and their entire removal from some 

 places by denudation, the north-west of Scotland, and probably a 

 large tract lying around it, sank under the sea. The depression 

 seems to have been slow and gradual, and to have continued until the 

 site of the Cambrian basins and of the surrounding region was covered 

 with a considerable depth of clear open sea-water. The records of 

 this subsidence are contained in a series of strata having a total 

 thickness of somewhere about 2000 feet, and divisible into two chief 

 groups — a Lower, composed of quartzitcs, grits, and thin con- 

 glomerate, about 500 feet in total depth, and an Upper, consisting 

 almost wholly of limestone. Perhaps the most striking feature in 

 this series of stratified rocks is the abundance of their organic remains. 

 The quartzites are crowded with the tubes formed by sea-worms when 

 the material existed as soft white sand on the sea-bottom. The lime- 

 stones are made uj) of the remains of calcareous organisms, among 

 which the most conspicuous that now remain are chambered shells 

 and gasteropods. Throughout these limestones worm-casts are present 

 almost everywhere, and in such abundance as to show, as Mr. Peach 

 has pointed out, that " nearly every particle of the calcareous mud 

 must have passed through the intestines of worms." A large collec- 

 tion of fossils has been made by the Geological Survey from these 

 limestones, which, though not yet specifically determined, amply con- 

 firm the original generalisation of Salter, made more than thirty 

 years ago, that the aspect or facies of organic remains in the lime- 

 stones of the north-west of Scotland resembles that of the older parts 

 of the Lower Silurian formations of Canada rather than that of the 

 corresponding rocks in Wales. So marked is the resemblance to the 

 American type as to indicate that some shore-line must once have 

 stretched across the North Atlantic, in order to afibrd a platform for the 

 free migration of marine life between the two areas. The contrast 

 with the Welsh type has been explained by the probable existence of 

 some barrier that separated the sea-bed over the north-west of Scotland 

 from that of southern Scotland, England, and Wales. That such a 

 barrier existed is tolerably certain, and I shall presently refer to some 

 indications of its probable position. At the same time it may be 

 open to question whether the Durness limestones can be properly 

 correlated as homotaxial equivalents of any Lower Silurian rocks in 

 Wales. My own impression is that they may be older than the 



