464 THE OLD RED SANDSTONE OF THE CLYDE AREA. 



are due (as the writer suggested some years ago) to the reducing action of 

 organic matter upon aqueous solutions of sulphate of lime, and mark a 

 transition-period from the extremely arid climatical conditions which pre- 

 vailed during the earlier part of the period to the humid conditions which 

 characterised the succeeding Carboniferous times. Everywhere the Upper 

 Old Red lies with a strongly-marked unconformity upon the rocks below, 

 including the granites which are contemporaneous with the Caledonian Old 

 Red volcanic rocks. 



The Upper Old Red Sandstone includes no volcanic rocks. It has yielded 

 fossils at a few localities within the basin of the Clyde, chiefly at the Heads 

 of Ayr. 



There is some reason to believe that part of the rocks within the Clyde 

 area which have been referred to the Upper Old Red Sandstone may really 

 belong to the Bimter division of the New Red rocks. 



