DELESSE ORIGIN OP BOCKS. 11 



closely to granite in its mineral composition ; for its constituent 

 minerals occur in granite, and moreover its accessory minerals are 

 also there met with. It has sometimes a well-developed crystalline 

 structure ; its metamorphism is analogous to that of granite ; and it 

 sometimes passes insensibly into that rock, with which it is very often 

 associated. 



" I think then that diorite is formed under conditions interme- 

 diate between those producing trap and granite ; but in the gene- 

 rality of its characters it is essentially allied to granite, and conse- 

 quently it has been chiefly engendered by water and pressure, the 

 part played by heat being very subsidiary. Kersantite and euphotide 

 may have had a similar origin ; whilst hypertite and melaphyre tend 

 towards trap, and even basalt, conducting thus the transition into 

 the volcanic rocks." 



§ 26. Serpentine. — More inexplicable than all other eruptive rocks, 

 serpentine remains to the present hour a perfect enigma. 



The preceding investigations have completely familiarized us with 

 the idea that an eruptive rock may be hydrated. If in the pseudo- 

 igneous rocks, such as retinite and basalt, water and heat have played 

 the principal parts ; in serpentine the effects due to heat have nearly 

 disappeared, so much so that the plasticity of the rock can hardly be 

 otherwise attributed than to water and pressure. 



In conclusion, serpentine is. very tender and highly hydrated ; it is 

 associated with diorite and euphotide, into which it passes insensibly. 

 I think then that its origin is the same as with these. 



§ 27. Resume. — The chemical composition of rocks having very 

 different external features may be the same ; for the characters which 

 are proper to them not only depend on their composition, but still 

 more on the agencies which were exerted at the period of their for- 

 mation. We can thus understand how rocks having the same com- 

 position, and yet different from each other, have been produced at the 

 same geological epoch. One comprehends also why, reciprocally, a 

 like rock has been erupted at different epochs. 



The origin of rocks has given rise amongst geologists to inter- 

 minable discussions, in which the most opposite doctrines have appa- 

 rently triumphed in their turns. These fluctuations, sometimes very 

 abrupt, are explained by the exclusive importance attached to one or 

 other of the agents which have combined in the formation of rocks. 

 It is assuredly very difficult to make out the part which each agent 

 has played ; but it seems to me that the developments into which I 

 have entered lead one to repeat, with M. Agassiz* : " After all, the 

 truth, as it so often happens, is here found to be between the different 

 systems. The different agencies to which each party refer the results 

 have all exercised their influences." [W. J. H. and T. R. J.] 



* " Systeme Grlaciaire. Nouvelles etudes et experiences," l re purtie, p. 572. 



