OP VOLCANIC ROCKS. 77 



porphyry (rhyolite) cannot be distinguished under the microscope in any respect what- 

 ever from felsitic porphyry. If one dare not doubt the eruptive origin of the former, 

 one cannot but acknowledge the same mode of origin in regard to the latter." And 

 again: "Let us here remark, that the microscopic structure (texture) of trachytic 

 quartz does not differ in any respect from that of granitic quartz ; there is no difference 

 between these two families of rocks so widely separated in time, either as regards the 

 number or the appearance of the water and glass cavities." Considering the second of 

 the before-mentioned assertions, it is true that Mr. Sorby has, by his sagacious obser- 

 vations, come to the conclusion that granite solidified at a temperature of about 600 

 Pahr., and that the various granites have been formed under a pressure equivalent to 

 a depth of from 40,000 to 69,000 feet. The calculation by which these figures were 

 obtained, is made upon the basis of the proportion between the bulk of the water 

 contained in the microscopic cavities, and the size of the vacuity, the latter being sup- 

 posed to indicate that the water had formerly filled the entire cavity and contracted 

 within it after the solidification of the surrounding rock. The laws of hydrostatic 

 pressure, however, as Daubree justly remarks, are in such a case not applicable in the 

 same way as they would be in a column of water ascending through a fissure ; temper- 

 ature and pressure may, in a mass of rock solidifying from a viscous state, be pre- 

 served, as in a closed vessel, to within a few feet from the surface ; "it is, therefore, 

 possible that many processes, such as the crystallization of granite, may have been 

 going on under pressure, though at a very limited depth." Considering that a mass 

 of granite, after the solidification of its exterior portion, will indeed be enclosed within 

 walls that may offer a strong resistance equivalent to a great pressure, it may be in- 

 ferred that it will assume, in crystallizing, a similar texture to that which it would 

 have obtained if the same process had been going on at a great depth below the 

 surface. It can hardly be suggested what must have been the texture of the crust of 

 a granitic mass. But the age of the granite is sufficient to justify the conclusion that 

 the crust of those masses which solidified on the surface of the globe must have been 

 completely abraded, and only those portions be preserved to observation which con- 

 solidated under considerable pressure. 



Though these arguments may show that the difference in the character of a 

 large mass of rock need not necessarily be proportionate to the depth under the sur- 

 face of the globe at which it solidified, and that granitic and volcanic rocks are nearly 

 related as regards their microscopic texture ; yet, the conspicuous external differences 

 between these two classes of rocks must remain a problem difficult of solution. The 

 proportionate rapidity with which volcanic rocks, on account of their usually small 

 volume, must have cooled, may be among the reasons. But it is not the only one. There 

 are differences in the nature of rocks, which escape our present means of explanation. 

 Hornblende-propylite, though no doubt at all can be entertained in regard to its sub- 

 aerial solidification, has completely the character of the so-called Plutonic rocks, and 

 yet lacks their supposed principal distinguishing mark : crystallization in depth. Con- 

 sidering that certain varieties of propylite, andesite, and trachyte are modifications of 

 the same group of chemical compounds ; and, though having been solidified on the sur- 

 face of the globe within a short period, yet exhibit marked external differences, we can- 

 o (115) 



