The Geology of the Cluden Basin. 131 



Birkhill shales. During the Llandovevv age, when these shales 

 were being laid down, this district was just inside the area of 

 sedimentation, and therefore only a \er\ fine mud was deposited 

 here during the whole age ; while at Girvan the same interwil of 

 time accumulated a coarser deposit over 1000 feet thick. The 

 Birkhill shales crop out in the Glenhurn, near the Scaur 

 Farm, Shawhead. Their fossils proclaim a strange monotony of 

 animal iifc. The graptolites, floating about in the ocean, had 

 migrated from the Skiddaw district during a prex'ious age. 

 After a long period of evolution, they developed into the form of 

 Monograplus priodon. Although the\ had now reached a higher 

 stage of animal life, yet the race was rapidly dying out in indi- 

 viduals, and also in genera and species. As the Tarannon age 

 drew near, the sea bottom was slightly raised until the coarse 

 earthy materials swept over the ocean's floor, and built up the 

 massive deposit of Queensberry grits. These muddy seas were 

 now so unfavourable to the graptolites that they speedily dis- 

 appeared — probably following the famous ad\ice " to baud 

 sooth." The type fossil of the grits is Monogr turrirulatus, but 

 it is very rare. The bands of conglomerate at Craigenputtock, 

 Bogrie, and elsewhere prove that the land was towards the north- 

 west, and that the sediment came from shores in that direction. 

 At the close of the Tudlow age the sea retired from this localitv, 

 and the land was gradually elevated into a plateau. All these 

 different layers of strata were originally laid down in a horizontal 

 position, but new forces soon began to play upon the elevated 

 land. Great lateral compression was exerted on the Silurian 

 strata, probably caused by contraction of the earth's crust. As 

 a result these deposits lost their horizontal bedding, and were 

 thrown into numerous folds and o\'erlaps, extending from a few 

 feet to several miles. This folding of rock was not caused bv 

 any sudden or volcanic action, but bv a gradual and imper- 

 ceptible movement. A very common form in the Gallo\vav hills 

 is that of an anticline or arched structure, while a composite form 

 is that of an anticlinorum. As the ages rolled on, denudation 

 proceeded in its usual slow but irresistible course. By atmos- 

 pheric influences, river action, and other agencies, the contour of 

 the land was completely altered. The crests or arches of the 

 anticlines were frequently removed, thus laying bare the under- 

 lying older deposits. This has taken place on a large scale in 



