﻿168 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 5, 



Whilst the Scottish Silurians often differ in aspect from the typical 

 rocks of the same age in central England and Wales, many of them, 

 however, bear a strong resemblance to their nearest equivalents of 

 Cumberland and Westmoreland ; and the strata in Kircudbright Bay- 

 may very well be compared with some of the Ireleth slates, described 

 by Sedgwick*. I never shall forget my surprise when I first satisfied 

 myself ' in situ,'' by inspecting their fossils, that the hard greywacke- 

 flagstones of Kirkby Lonsdale were the equivalents of the Ludlow 

 rocks, and that the calcareous slates of Ireleth were the true repre- 

 sentatives of the soft Wenlock formation of Shropshire. Still less 

 could I or any one else have supposed, that the incoherent plastic clay 

 of the banks of the Neva and the green-grained rocks of St. Peters- 

 burgh (the " craie chloritee " of Brongniart) would prove to be as 

 old as the slates of Snowdon ! 



Lastly, it appears to me certain, that all the above-mentioned 

 varieties of greywacke, clay-slate, conglomerate, sandstone, and lime- 

 stone of the southern Scottish counties constitute parts of one system 

 only, and that, physically as well as zoologically, they are all knit 

 together. Although full of local dislocations, folds, and numerous 

 short axes of elevation, these deposits followed each other in seas 

 tenanted by the same classes of animals, some in deep, others in shallow 

 water, — some in a period of tranquillity, others during very powerful 

 abrading action. In the prevalent absence of lime, and of mineral 

 conditions favourable to the life or preservation of testaceous marine 

 animals, the Silurian deposits of Scotland long remained in obscurity ; 

 but now, thanks to the opening-out of certain shelly oases within them, 

 we are enabled to read off their history and to compare them with 

 primaeval types established by the survey of other regions. 



End of Part I. 



In a second part of this Memoir, read before the Geological Society 

 (February 26, 1851), I endeavoured to show from a section, made ten 

 years ago, that the northern portion of the Sidlaw Hills in Forfarshire 

 would be probably found to contain strata equivalent to the Uppermost 

 Silurian or Ludlow rocks. That is one of the tracts of Scotland, in 

 which, according to my notion, the lowest division of the Old Red 

 Sandstone is most copiously developed, and there also rise out inferior 

 beds of grey, micaceous sandstone and flagstone, some of them un- 

 distinguishable from the Ludlow rocks of England, and like them 

 containing forms of the fossil crustacean Pterygotus, known to the 

 workmen of the Scottish quarries under the name of the "Cherubim." 

 In explaining this point, I referred to a report of a discovery (which 

 seems to me to be very probable, but has not been confirmed) that 

 Graptolites and other Silurian 1 fossils were found in the same tract, 

 i. e. in a cutting of the Newtyle railroad. 



I next described the symmetrical arrangement of a great anticlinal 

 axis in the south-western Highlands, which I observed last summer, 

 * Proc. Geo]. Soc. vol. iv. p. 580. 



