THE MICROSCOPE IN GEOLOGY. 
361 
silver smelting, and is here given for the sake of comparison with 
the structure of eruptive rocks. In formation it is so nearly 
identical with what is seen in sections of more felspathic basaltic 
rocks, the mass of which consists of a framework of interlaced 
crystals of labradorite with the interstices filled up with the 
other mineral constituents confusedly crystallised, that this 
section might easily be mistaken for such. The Eowley rag, 
when fused and very slowly cooled, presents a similar appear- 
ance ; and, in general, the structure of crystalline slags presents 
many features in common with that of ordinary eruptive rocks. 
Before proceeding to the next class of rocks, the discovery by 
Sorby of the numerous minute fluid cavities in the quartz of 
granites should be alluded to, as proving the great value of 
the microscope in the study of these rocks. The result of this 
gentleman’s researches * proves 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 (J to J per cent.) 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 positive of the non-igneous 
origin of granite ; whereas if Mr. Sorby’s memoir had actually 
been read, it w T ould have been seen that he had found fluid 
cavities, perfectly identical with those in 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 cavities, 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 agencies. He 
therefore classes them together under one head as rocks of 
similar origin, f 
II. Secondary or Sedimentary Eocks. 
The rocks pertaining to this class are all, directly or indirectly, 
formed from the breaking up, or debris, of previously existing 
* Quart. Jour. Geol. Soc. vol. xiy. pp. 453 — 500. 
t These researches tend to confirm the theory of the igneous origin of 
granite and eruptive rocks in general. It must not he forgotten that by igneous 
action , as used by the Piutonist, was always understood the action of heat as 
developed in volcanoes (the study of which was the basis of the theory itself), 
in which the agency of water was always recognised. Nearly half a century 
ago, Scrope not only insisted on the important part played by water in vol- 
canic action, but specially pointed out the difference between such volcanic 
fusion and ordinary melting. The term hydro-igneous action might not be 
inappropriate for such, but hydro-thermalism does not at all express what is 
intended. The idea of a true dry fusion in nature exists only in the brains 
of the ultra-Neptunist or lukewarm hydrothermalist. 
