THE MICROSCOPE IN GEOLOGY. 1 1 9 



have proved satisfactorily its identity with the Rowley 

 basaltic rock." In external appearance, the mineral 

 uralite resembles augite, but its chemical composition 

 is that of hornblende ; the microscope distinctly re- 

 veals the fibrous structure characteristic of the horn- 

 blende. 



Some years ago, you may remember, a geological 

 heresy was maintained by some, that granite had not, 

 after all that had been said, an igneous origin. Let 

 us see what part the microscope played in determin- 

 ing the question. Mr. Sorby discovered in the quartz 

 of granites numerous minute fluid cavities, thus show- 

 ing that granites have solidified at a heat far below 

 the fusing points of their constituent minerals, and at 

 such a pressure as to enable it to entangle and retain 

 a small amount of aqueous vapour, which naturally 

 must have been present during its liquefaction. 

 " The presence of these fluid cavities in the quartz of 

 granite was immediately blazoned forth as proof posi- 

 tive of the non-igneous origin of granite ; whereas, if 

 Mr. Sorby' s memoir had actually been read, it would 

 have been seen that he had found fluid cavities per- 

 fectly identical with those of granite, not only in the 

 quartz of volcanic rocks, but also in the felspar and 

 nepheline ejected from the crater of Vesuvius; and 

 that the presence of fluid, vapour, gas, and stone cavK 

 ties are common both to the volcanic quartz-trachytes 

 and to the oldest granites ; and the inference drawn 

 by Mr. Sorby from the results of his researches, is 

 that both these rocks were formed by identical agen- 

 cies." As with regard to- the volcanic, so with the 

 sedimentary rocks ; a microscopic examination alone 

 will afford correct information as to their origin; but I 

 must refer you to Mr. David Forbes' most interesting - 

 memoir for further details. Mr. D. Forbes gives 

 the following instructions how to prepare rock sec- 



