284 STEEAM '11$ DEPOSITS OF PEBAK. 



in granite paste. But the most extraordinary thing of all is 

 that fossils have been found in granite, much, changed, of course, 

 and crystalline, but perfectly recognizable. The Jura Belemnites 

 in the Alps may be cited as an example, and I think I have met with 

 paleozoic fossils in a granitic rock in Australia. 



All this was very puzzling and gave rise to many theories. 

 The facts seemed to hold the balance equally between a stratified 

 rock on the one hand, and a kiud of volcanic, or at any rate, an 

 eruptive rock on the other. Then the theory of metamorphism 

 began to make its way. This suggested the granite had origiually 

 been a stratified rock, and that it had been converted into its pre- 

 sent form by the agency of heat. 



This, you observe, only removed the difficulty one step further 

 back. The question was still unsolved as to what kind of heat it 

 was. Gradually the microscope was brought to bear upon the 

 matter, and this, with chemical aids, brought what is now believed 

 to be a full and satisfactory explanation. 



If yon subject a small rough fragment of granite to micros- 

 copic examination, you will not learn much. But if you grind 

 down thin polished slices until they become quite transparent, 

 you will be able to subject them to very high magnifying powders. 

 Then you will see that the apparently solid crystals are full of 

 minute cavities. Some of these are partly filled with water, 

 others with gas. others again are cavities containing j)erf ect crystals 

 of such minerals as common salt, and other salts of magnesia, soda, 

 &c. These crystals sometimes appear in fluid, which may be 

 water, and they are in constant movement. 



It would be an error, however, to suppose that these appear- 

 ances are only found in granite crystals. They are seen, though 

 not exactly in the same manner, in volcanic rocks, in meteoric 

 stones and even in the slags of furnaces. But microscopic examina- 

 tion has shown immense differences between granites and those 

 which have been certainly subjected to heat within reach of the 

 earth's atmosphere. 



I cannot, in the limits of such a lecture as this, go into the de- 

 tails of this subject, but it will be sufficient to say that the progress 

 of science, largely aided by the microscopic investigation of rocks, 

 has shown us a most probable and sufficient cause for the meta- 

 morphism of granites. All the different effects of heat are found 

 to vary according to the pressure at which they have been exer- 

 cised. 



It will save a great deal of explanation if I enter at once 

 into the consideration of what must have been the geological his- 



