Formation and Age of the Queensberrys. 253 



and 1200 at Abington; their equivalent of deep-sea deposit at 

 Glenkiln is only 20 feet. The next age, termed Caradoc, is made 

 up of the Hartfell black shales and the Barren mudstones above. 

 The strata contain occasional flows of lava, which mark the close 

 of volcanic activity during the Silurian epoch. The Lower shales 

 show no change of ocean depth, but the Upper mudstones point 

 to a slight oscillation in the sea floor. The whole Caradoc Age 

 gives us a total thickness in Queensberry area of 100 feet; Lead- 

 hills and Elvanfoot show 1800 feet; while Girvan strata in same 

 age amount to 2800 feet. Coniston Old Man, in the Lake Dis- 

 trict, and Snowdon, in North Wales, belong to the same age. 



These three ages already dealt with were previously termed 

 Lower Silurian — reserving the term Upper Silurian for those to 

 follow; but the comprehensive change, as evidenced by the 

 fossils, between the two periods has demanded a greater distinc- 

 tion of terms: Lower Silurian — Ordivician ; Upper Silurian now 

 means Silurian. 



The Birkhill shales usher in this new Silurian period with an 

 age of their own — Llandovery. The deep-sea conditions are still 

 very similar, giving 98 feet of Birkhill shales against 1000 feet 

 coarser strata at Girvan ; but the absence of volcanic matter and 

 the vast change in the life of the ocean fix the lowest zones of 

 these shales as marking the boundary line between the Ordovician 

 and the Silurian. Towards the top of this group there is evidence 

 of a marked change in the ocean floor. The shales are gradually 

 replaced by coarser sediments, known as the Queensberry grits, 

 and assigned to the Tarannon Age. An oscillation in the earth's 

 crust has brought the shore considerably nearer, giving us in this 

 age 4000 feet of strata against 2000 feet in the Girvan area. 

 These grits of Tarannon Age are the highest strata now visible in 

 the Queensberry structure, but the period of sedimentation did not 

 cease at this point, but continued during three more ages till the 

 close of the Silurian epoch. As a result of the three last ages of 

 continued ocean, fresh sediments were accumulated above the 

 Tarannon grits. It is impossible to say what the original thick- 

 ness may have been, but there still remain isolated fragments 

 proving at least a thickness of 5000 feet. Finally these masses of 

 Ordivician and Silurian strata, representing the accumulations of 

 eight ages, were elevated, forming one vast tableland. Probably 

 the same power that caused their elevation would also account for 



