122 T. Sterry Hurt on Granitic Rocks. 
previously rounded and enclosed in a large crystal of quartz, 
t r ich are also nearly obliterated. From the 
alternations in the deposited mineral matters in many veinstones, 
as well as from what we know of the changing composition of 
mineral springs, it is evident that the waters circulating in 
fissures now occupied by veins, must have been subject to 
periodical variations in compositio 
me of the more important types of Laurentian vell- 
stones may now be noticed. Those made up of quartz wl 
orthoclase, muscovite and black tourmaline, often with zircon, 
are not unfrequent in the Laurentian gneiss, though so far a8 
yet observed less abundant than in the gneisses and mica-schists 
of the White Mountain series. It is true, as already pointed 
out, that from the greater softness of the enclosi the 
and are thus rendered more conspicuous than those in the 
mates of the same author is an opalescent or chatoyant 
ite which occurs with quartz in another vein in the same reglo™ 
closing rocks 
veins of the latter series are often weathered into relief (§ 20), 
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