TlElA.N&A^CTLO:S&. 



The uu-thors of the followhuj papers ore alone responsible for the 

 opinions expressed : — 



THE ORIGIN OF THE PERMIAN BASIN of THORNHILL. 



By Joseph Thomson, Gatelawbridge. 



Read 2fl February, 1877. 



(1.) Superficial Position and Area of the Permian 

 Rocks of Thornhill. 



Those of you who have travelled through the middle 

 ward of Nithsdale, m the centre of which Thornhill stands, 

 wall have observed that it is a small valley in itself formed 

 by hills of Silurian rock, which suiTOund it on all sides, and 

 from which in a former era of the world's history it has been 

 worn out by some denuding agent. At the bottom of this 

 small valley lie the rocks which are to form the subject of 

 our inquiry to-night. They extend a distance of 12 miles 

 from the low-lying hills which bound the valley on the south 

 to the mouth of the Pass of Dalveen — in fact the extreme 

 length of the valley. A line running from east to west 

 through Thornhill will lie along its greatest breadth, which 

 is about 4 miles. That these rocks are referable to the 

 Permian system is inferred from the following facts :— 1st, 

 They overlie the Carboniferous system unconformably ; 2d, 

 they are to a large extent identical with strata which uncon- 

 formal:)ly overlie the Coal measures in Ayrshire ; 3d, they 

 are the same as those which, in the Dumfriesshire basin, pass 

 soutliward under the Trias of Cumberland. 



(2.) Charactemistics of the System. 

 Perhaps few places in Great Britain present a more 

 interesting development of the features which so peculiarly 

 characterise the Permian strata as the little basin of Thorn- 



