Professor Geikie on the " Pitchstone" of Eskdale. 253 



as ranging between 50 and 55, though in some varieties it 

 sinks below the lower limit, and in others rises above the 

 higher. In the series of basalts collected during the Geologi- 

 cal Exploration of the 40th Parallel in the Western States 

 and Territories of the American Union, the percentage of 

 silica varies from 48*4 to 54-8.* 



But if w^e pass to the vitreous types associated with the 

 basalt rocks we encounter a still higher proportion of silica. 

 Thus in the pitchstone-like external crust (0*03 metre thick) 

 of a dyke (0-2 metre broad) in Lamlash Island, Arran, 

 Delesse found the composition to be : silica, 56*05 ; alumina, 

 1713; peroxide of iron, 10-30; protoxide of manganese, a 

 trace; lime, 6-66; magnesia, 1'52 ; potash, 0-98; soda, 3-29 ; 

 water, 3*50 = 99-43.i- He found the composition of the 

 crystalline portion of the dyke to be nearly the same, but 

 the specific gravity to be 2-649, while that of the glass was 

 2-714. 



At my request my colleague, Mr J. S. Grant Wilson, has 

 subjected the Eskdale dyke to chemical analysis. The 

 specific gravity of a normal portion of the glassy rock is 2-7. 

 An average sample of the same rock was reduced to powder, 

 and allowed to remain in hydrochloric acid for ten days. It 

 was thereafter boiled. At the end of this treatment the pro- 

 portion of the rock dissolved in the acid was found to be only 

 16*8 per cent. 



Another portion of the powdered rock submitted to quan- 

 tative analysis gave the following results : 



100-11 



* Explor. 40tli Parallel, vol. i. ; Table of Analysis, xii. 

 t Ann. des Mines, xiii. (1858), 369. 



