so 



than the Carboniferous, the small grains of quartz are specially 

 interesting, because, in some parts of the sandstone, at any rate, 

 each grain shows the form of a doubly pyramidal crystal, the 

 crystals being of very uniform size, and their form often not a great 

 deal affected by roUing. Whence these little crystalline particles 

 could have been derived, to form the sandstone, is somewhat of a 

 puzzle. Among the Blue Mountains of New South Wales, Darwin 

 observed a similar case, and remarks, " It is difficult to imagine 

 how these crystals can have been formed ; one can hardly believe 

 that they were separately precipitated in their present crystallized 

 state. Is it possible that rounded grains of quartz may have been 

 acted on by a fluid corroding their surfaces and depositing on them 

 fresh silica?" The silica in old glass sometimes regains its 

 crystalline structure, as shown by Sir David Brewster in 1840. 



(b) Quartz as a Constiiiunt of Vnstratified and Volcanic Rocks. 

 :. Among Granitic and Granitoid Rocks. 

 Generally speaking, in true granites, quartz occurs uncrystal- 

 lized, being the last of the constituent minerals to solidify ; it then 

 appears to fill up all the interstices of the other minerals, the 

 felspar and the mica. This is the case with the Skiddaw, Eskdale, 

 and Shap granites in the mass, though sometimes very locally or 

 in the form of dykes proceeding from the main mass, the disposi- 

 tion of the quartz is not interstitial but crystalline. This latter mode 

 of occurrence may be well studied in the Quartz Felsite of St. John, 

 and notably in the Armboth Dyke. In these cases the quartz and 

 some of the felspar have crystaUized out in a felsitic base, and the 

 cross sections of the quartz crystals often look nearly square, from 

 the unequal development of all the six sides. In the Armboth 

 Quartz Felsite Dyke the embedded crystals are most clearly seen, 

 and the pyramidal termination of both ends may be well observed. 



2, Among intrusive Diorites, Dolerites, &'c. (Greenstones. ) 



I know of no cases of quartz occurring in a crystalline 

 condition in the rocks of the Greenstone class. Nor is quartz 

 generally a conspicuous constituent in any form among such rocks. 



