254 Proceedings of the Royal Physical Society. 



Some of the isolated kernels or particles of black glass dis- 

 persed through the rock were picked out and found by Mr 

 Wilson to yield 65*49 per cent, of silica; with alumina, 

 14-66; protoxide of iron, 5*44; lime, 3-73; magnesia, 1"57; 

 the alkalies, owing to the small quantity of the material 

 available, were not determined. 



In its specific gravity and high proportion of silica the 

 Eskdale rock agrees with other known vitreous basalts. It 

 is worthy of remark also that its glass kernels contain about 

 7 per cent, more of this acid than the general mass of the 

 rock. 



Comparing its characters with those of the various hyalo- 

 melans and tachylites which have been described by Zirkel, 

 Mohl, Fischer, Eosenbusch, and others, we observe that it 

 differs from all of them as they in turn differ from each other. 

 If we follow the not very satisfactory classification which 

 ranges as tachylite those glassy forms of basalt, which are 

 more or less readily soluble in acid, and, as hyalomelan, those 

 which resist the acid, we see that the Eskdale rock takes an 

 intermediate position, rather inclining to hyalomelan. Yet 

 its internal structure, abounding in microliths, but with no 

 spherulitic concretions or gas-pores, links it rather with the 

 described forms of tachylite. I have recently had sections 

 prepared of other vitreous basalts from different parts of 

 Scotland, and hope in another paper to bring forward addi- 

 tional information regarding this interesting and still imper- 

 fectly understood condition of our basalt-rocks. From what 

 has now been laid before the Society, it will be seen how 

 much more detailed must be the work of the geologist and 

 the petrographer than was possible in the days of Jameson, 

 and how vastly greater are now the facilities for this research. 

 There can be no doubt that the rocks of Scotland present a 

 marvellously rich field for the application of modern methods 

 of petrography. 



Explanation of Plates. 



Plate V. 



Fig. 1. Section of Eskdale dyke near Eskdalemuir Manse : a, a, Highly 

 inclined Silurian greywacke and shale ; h, b, Dolerite, c, vitreous basalt of 



