English Formation^. 303 



and in South Wales skirt the Bristol Channel, and 

 stretch into the interior in Pembrokeshire, Glamor- 

 ganshire, and Monmouthshire ; while in the north 

 they border North Wales, and form a broad backbone 

 of country that reaches from the borders of Scotland 

 down to North Staffordshire and Derbyshire. Other 

 patches, here and there, rise from below the Secondary 

 strata into the heart of England. (See Map.) 



The general physical structure of England, from 

 the coast of Wales to the Thames, will be easily under- 

 stood by a reference to fig. 57, p. 304, and to the fol- 

 lowing descriptions; and this structure is eminently 

 typical, explaining, as it does, the physical geology of 

 the greater part of England south of the Staffordshire 

 and Derbyshire hills. 



The Lower Silurian rocks of Wales (No. 1) consist 

 chiefly of slaty and solid gritty strata, accompanied 

 by, and interbedded with, numerous felspathic lavas 

 and beds of volcanic ashes, marked -f ; and mingled 

 with these there are numerous bosses and dykes of fel- 

 stone, quartz-porphyry, greenstone (diorite), and the 

 like. These last, by their superior hardness, give a 

 mountainous character to the whole of North Wales, 

 from Merionethshire to the Menai Straits. In part of 

 north Pembrokeshire also, in a less degree, igneous 

 rocks are largely intermingled with the Lower Silurian 

 strata, and these, by help of denudation, now form a 

 very hilly country. 



Without again entering into details, it is here 

 sufficient to state that the Cambrian and Lower Silurian 

 epoch was ended in the British area by disturbance and 

 contortion of the strata, and their upheaval into land. 

 This disturbance necessarily gave rise to long-continued 

 denudations of this early English land, both by ordinary 



