244 Proceedings of the Royal Physical Society. 



nised that the old methods of study are inadequate. They 

 admit that a chemical analysis, on which they used implicitly 

 to rest, can after all tell us only the chemical elements of a 

 rock or mineral. What may be the combination of these 

 elements must be a matter of inference, unless the actual 

 internal structure of the rock can be scrutinised. But it is 

 here that the microscope comes in to our aid, laying open to 

 the eye the forms in which the chemical constituents 

 have been combined, and the structure of the rocks 

 which they compose. Hence as questions of structure 

 necessarily involve questions of origin, the genesis of rock- 

 masses can often be followed with singular clearness by 

 microscopic research. Moreover, all rocks exposed to the 

 influence of the atmosphere or of percolating water, are 

 subject to more or less alteration. In the microscope we are 

 furnished with an admirable implement for tracing these 

 changes from their first beginning onward until the total 

 obliteration of original structure and composition. 



I offer here the results of a microscopic examination of the 

 Eskdale dyke, first of the doleritic external band and then of 

 the vitreous centre. 



(1.) Microscoijic Structure of the Dolerite forming the 

 marginal hands of the dyke. 



I have already remarked that the dolerite on either side of 

 the Eskdale dyke shows the usual crystalline character, 

 except towards the line of contact with the surrounding 

 greywacke and shale, where, at the Shaw Burn and else- 

 where, it presents the close-grained texture commonly to be 

 observed in an intrusive mass along its contact with the 

 rocks which it has invaded. The general character of this 

 fine-grained, or chilled, edge of the dolerite, as seen under 

 a magnifying power of forty diameters, is represented in 

 fig. 1 of PL VL There can be no doubt that the marginal 

 portions of dykes and intrusive sheets represent earlier 

 stages in the solidification of the once molten masses. Chilled 

 by contact with the surrounding rocks, the basalt or dolerite 

 was rapidly congealed; while further away, where cooling 

 was more prolonged, the conditions were more favourable 



