32 PALEOZOIC TIME. [CHAP. III. 



posits were accumulated to a thickness, in parts, of at least 

 12,000 feet, and the highest beds known are unsucceeded 

 by any conformable series of sedimentary rocks ; hence we 

 know not how much of the products of the old volcano has 

 been lost, and for aught we know to the contrary, an Etna 

 in size may once have stood where now are the resting- 

 places of quiet lakes." 



When volcanic activity had ceased, " a subsidence of the 

 region ensued, accompanied doubtless by much waste of 

 the volcanic material through the agency of atmospheric 

 denudation. Subsidence, however, continued until the old 

 volcano came within the planing power of marine coast- 

 action, and at last there was probably but little of the old 

 terrestrial volcano left above the level of the sea." As the 

 waters gradually crept over the site of the volcanic distur- 

 bances the calcareous sediment which was being deposited 

 on the surrounding sea-bottom gradually enveloped and 

 covered the surface of the sinking volcanic area. That this 

 sea covered the greater part of northern England is known 

 from the existence of Ordovician rocks in Teesdale and 

 near Ingleborough. 



Scotland. In the south of Scotland Ordovician rocks are 

 known to occupy large areas, and they have been mapped 

 by the Geological Survey, but little information has yet 

 been published about them. There are, in fact, only two 

 districts where the vertical succession has been ascer- 

 tained, viz., Girvan in Ayrshire, and Moffat in Dumfries, 

 and our knowledge of these is almost entirely due to the 

 careful investigations of Professor Lap worth. 



It so happens that though the same succession of fossils 

 can be traced in each area, yet the characters and thick- 

 nesses of the beds containing them differ enormously, the 

 Girvan strata being over 3,000 feet thick, while those of 

 Moffat are only 150. 



In the Girvan area there are shales of Arenig age, but 



