President's Address. 



7 



tinued heat on metals and minerals, says — "The crystalli- 

 zation of the primary rocks was supposed by the early 

 Plutonic theorists to be due to slow cooling ; but this 

 principle alone does not satisfy the phenomena. The crys- 

 talline structure of granite is seen, for example, in Glen 

 Tilt, at Shap Fell and elsewhere, to be equally uniform in 

 its partial irruptions into the superior strata, as where it 

 appears to be the foundation-stone of the earth's crust it 

 has crystallized in its accustomed manner, where it has 

 penetrated fissures of the upper beds in plates as thin as the 

 leaves of a book, and threads as fine as a hair ; and even 

 where it is involved in the invaded stratum, so that no junc- 

 tion with any vein can be observed. How could it have 

 been thus injected in a state of fusion unless of the most 

 liquid kind ? and how could the heat of such liquidity, in a 

 material of which the fusing point is so high, be otherwise 

 than rapidly cooled down ?" Yet this difficulty does not 

 prevent the reverend gentleman from still believing that 

 the granite was of igneous origin, and persists in his belief 

 in the face of the fact, that " the quartz which forms so 

 large a constituent of granite has always the specific 

 gravity of crystalline silica, which exceeds that of any other 

 species of silica ; and Deville and others have shown that 

 fusion lowers this specific gravity to a constant amount, and 

 that fused silica does not recover its density in cooling. 

 Crystalline granite, as Delesse has shown, passes by fusion 

 from the density of 2*62 to that of 2*32, and Egyptian 

 porphyry from 2 '76 to 248." 



The Plutonists, in their anxiety to maintain the old doc- 

 trine of internal heat, have not, in my opinion, improved 

 their position by discovering that the ramified granite of 

 Glen Tilt and Shap Fell is similarly crystallised with the 

 rest of the rock, but only finer grained ; nor has the ex- 

 periment of Mr Marshall, who fused a large mass of granite, 

 and having slowly cooled it, failed to obtain crystals, aided 

 their hypothesis. 



If the experiments of Hall were correct, and the specu- 

 lations of Hutton true, we ought to find, wherever the gra- 

 nites are intruded, metamorphic changes due to heat m ade 



