XV. Observations on the Structure of the Border Country of Salop 

 and Noi^th Wales ; and of some detached Groups of Transition 

 Rocks in the Midland Counties. 



By the Rev. JAMES YATES, MA. F.G.S. 



[Read March 4th and 18th, 1825.] 



Of the rocks which occupy the confines of Shropshire and North Wales, 

 lying- between the slate mountains on the West, and the extensive formations 

 of limestone, red sandstone, marl, and gravel, on the East, it may be observed 

 generally, that they are very various in their character, and differ exceedingly 

 in their thickness, their induration, and their colour. They are sometimes 

 earthy and amorphous, and at other times crystalline, in their structure. They 

 have sometimes the appearance of mechanical, sometimes of chemical depo- 

 sits, and sometimes these appearances are united in the same rock. In their 

 composition, they pass from argillaceous to siliceous, and from siliceous to 

 calcareous ; but the prevailing earth appears to be clay, the next in quantity 

 is silica, and carbonate of lime appears to be by much the least abundant of 

 the three. It may also be remarked, that these strata are of moderate thick- 

 ness, never perhaps exceeding 6 feet in depth, and varying from that to the 

 state of mere partings between one bed and another. The harder strata are 

 commonly separated by thinner beds of soft clay, and often divide, in conse- 

 quence of a kind of rude crystallization, into angular or rhomboidal pieces. 

 All these circumstances produce in them a manifest tendency to disintegra- 

 tion. They are generally conformable in their position, and most commonly 

 dip towards the East ; but occasionally they exhibit remarkable contortions. 

 They rise with a gentle inclination, and usually terminate towards the 

 West in abrupt escarpments, while the Eastern slopes of the hills, which they 

 constitute, correspond evenly with the surfaces of the strata underneath. They 

 are intersected also by tortuous valleys, which sometimes expand from the 

 condition of narrow wooded glens into level plains of great fertility. Of 

 the mountainous portions of this tract none perhaps are higher than 1500 



