T. Mellard Reade — British Geology. 563 



complex and difficult to trace than in South Britain, but it is 

 sufficient for the present purpose simply to mention them as all 

 pointing towards the immense range of geological time represented 

 by the conformable Upper Silurian Old Ked Sandstone and Carbon- 

 iferous of South-west Britain. 



Thickness and Extent of Boclc Formations a measure of their plication. 



Tlie preceding sketch is little more than the barest outline of 

 geological events recorded in the rocks of our own island, yet 

 attentively studied the following facts stand out prominently : — The 

 sedimentary rocks which were deposited in the greatest volume are 

 those also that subsequently became most plicated. Though they 

 were the earliest laid down they even now constitute the distinctly 

 mountain masses of Britain, and possess actually the highest peaks 

 and the highest average elevation. When it is considered that these 

 mountains have been exposed to the destroying agencies of denu- 

 dation for an enormous period of geological time, and still stand 

 pre-eminent above those built out of younger formations, we may 

 perhaps picture to oui'selves in some slight degree the extent of the 

 original mass. Not only are the Cambrian and Silurian formations 

 here of enormous thickness, but they are of surprising and unknown 

 extent, and they are found whenever denudation has proceeded far 

 enough to bare them to the eye. 



Again, the Carboniferous, taken together with the Devonian and 

 Old Red Sandstone, occupying the second place in the mountain 

 structure of the British Isles, consist of great masses of sediment, 

 not indeed rivalling the Silurian in volume, but of great extent and 

 thickness, though much denuded. Some portions are considerably 

 folded, and most are greatly faulted, but as a whole do not show 

 anything like the signs of lateral compression which are seen in the 

 Cambrian and Silurian. 



The Permian and Triassic rocks come next in time and, curiously 

 enough, in importance as regards thickness, and the country they 

 occupy, if not mountainous, possesses a more faulted structure than 

 any of the following formations, and is of a more hilly character. 



When we ascend to the Lias, Oolites, and Cretaceous, then the 

 beds become more continuous, horizontal, and less faulted. Folds, 

 and sharp ones, are to be met with, but these, as a rule, are local, 

 the lateral pressure to which they were subjected having been less, 

 and concentrated at fewer points. The same peculiarities apply to 

 the Tertiary rocks, perhaps in a stronger degree. 



Relation of Orographic Structures to. the mass of the sediment 

 composing them. 



I trust that I have now said sufficient to show that there is in 

 the British Isles an intimate connection between the depth, extent, 

 and mass of the several great deposits marking the progress of 

 geologically recorded events, and the structures into which they 

 have been severally raised. 



The mountain-building and the foldings and the faultings are, 



