T. Mellaril Reade — British Geology. 559 



That the Lias had formerly extended over a considerable area 

 of the western half is shown by the preservation of outliers in. 

 Shropshire, Staffordshire, and Cumberland, and prima facie it is 

 probable that the overlying formations have also at one time in 

 geological history been present. To a student of physical geology 

 a glance at the geological map of England is sufficient to show 

 that the outcrop of the Lias extending from the Severn to the Tees 

 is an escarpment of denudation. 



It must not be supposed for a moment that Britain represents 

 the normal physiographic condition of the various formations all 

 over the world, and that the mountain areas are always the oldest. 

 Quite the contrary. In the Alps, Himalayas, and Rockies, in the 

 mountains of the Caucasus and Central Turkestan, it is the younger 

 formations that constitute the highest members of the mountain belts, 

 and it is generally conceded that the bulk of the mountain-making 

 in these chains has been geologically a late event. 



Of course in great mountain chains we may have, and generally 

 do have, the older rocks, even to the Archean gneisses and schists 

 thrown up and exposed in their axial folds. This, however, as a 

 rule, only shows that the earth movements to which they owe their 

 birth and growth have been extreme and profound, involving the 

 deep-lying sediments, and even their foundation rocks, in the general 

 movement. We thus see that there is no intimate relationship 

 between the age of the rocks of which mountain areas are composed 

 and the existence of the mountains themselves. 



The Mountain Areas of England and Wales as related to the thicTcness 

 of the sedimentary and other deposits of ichich they are composed. 



The Lower Cambrian Rocks of South Wales are estimated to have 

 a thickness of from 3500 to 4000 feet, and are supposed to be over 

 8000 feet in North Wales. The Upper Cambrian, where well 

 developed, attain a thickness of from 5000 to 6000 feet. The 

 Ordovician, or Lower Sihirian of the Survey, is estimated at from 

 12,000 to 25,000 feet; in Shropshire, according to Murchison, 18,000 

 feet ; and between the Menai Sti'aits and the Berwyns 19,000 feet. 

 The Silurian, or Upper Silurian of Murchison and the Survey, may 

 be 14,000 feet in the north-west of England, and from 3000 to 6000 

 feet in Wales. The total thickness of British Silurians, Lower and 

 Upper, is stated by Murchison to be from 26,000 to 27,000 feet. 

 Thus, from the base of the Cambrian to the top of the Silurian cannot 

 be less than 35.000 feet. 



We thus arrive at the grand aggregate thickness of from six to 

 seven miles of rocks constituting the most mountainous parts of 

 England and Wales. These rocks have suffered more intense 

 deformation than any of the overlying formations, and have, as 

 first lucidly explained by Ramsay, suffered enormous denudation 

 since their folding and upheaval.^ 



1 " Denudation of South "Wales and the adjacent English Counties" : Memoirs 

 of the Geological Survey, vol. i. 



