238 Rev. James Yates on the Structure of the 



feet above the level of the sea. Various kinds of trap-rocks rise through 

 them in great masses, and are distinguished by their abrupt and rounded 

 summits. 



Passing from these general observations, we may endeavour to illustrate 

 the nature of the strata under review by the detail of their appearances in 

 particular places. This may be best done by pursuing the section of the 

 different rocks across their basset edges. 



1. To begin from the North and proceed gradually Southwards, the 

 line of section, which first claims attention, is in the vicinity of Vale Crucis. 

 The lofty Oernant Mountain rises above the northern boundary of the 

 valley of the Dee, and contains a large slate quarry, belonging to Sir W. W. 

 Wynne. Here the strata are curved with extraordinary regularity into the 

 form of concentric circles, the cleavages of the rock having also the ap- 

 pearance of radii, in consequence of being perpendicular to the direction 

 of the strata. These circumstances are deserving of notice, not only as 

 exhibiting a fine specimen of curvature, but as showing the primitive cha- 

 racter of the rock. 



At the distance of about 2 miles to the N. E. is presented the perpendicu- 

 lar face of the rocks of Edlw-y-seg. These are part of the edge of the great 

 basin of limestone, containing the coal formation of North AVales Their 

 direction is generally from East to West, but in a waving course, and through a 

 tract of many miles in length. A steep talus of debris covers about two thirds of 

 their height ; and above this their face rises almost perpendicularly, present- 

 ing a remarkable specimen of regular stratification, and a striking contrast to 

 the forms of the slate mountains in their vicinity. The divisions of the strata 

 of limestone are rendered conspicuous at a distance by beds of black bitumi- 

 nous shale, which lie between them. This shale is for the most part perfectly 

 soft and friable ; it is, however, frequently indurated by the addition of calca- 

 reous matter, and thus passes into foetid limestone. It is also sometimes mixed 

 with a large proportion of sand, and then resembles very much the mud depo- 

 sited in the estuaries of the Dee and Mersey. 



These cliffs contain various shells and corals, usually considered as belong- 

 ing to the mountain limestone of England. In the case of shells occurring in 

 the limestone beds, the matter of the shells is generally removed, only casts 

 remaining, or newer depositions of calcareous spar in place of the original 

 substance. But in the beds of shale the testaceous matter seems to be pre- 

 served with very little alteration in its texture or composition. In a formation 

 of so great antiquity in the geological history of the globe, this circumstance 



