FELDSPARS. 83 



and the only exception appears to be in the case of the anipliibolites, wliicli 

 seem most rationally regarded as extreme cases of the dioritic; group. The 

 genesis of feldspar in the metamorphic rocks is certain!}- one of the most 

 important changes, and it is also one which is very fully illustrated liy the 

 collections. The unquestionable authigenesis of minerals of this group 

 is excellently seen in No. 11, Knoxville, an augitic rock intersected by 

 minute, almost microscopic, veins. Portions of these veins are filled with 

 well developed, striated feldspar prisms and irregular grains of plagioclase, 

 seemingly oligoclase. In some portions carbonates are mixed with these 

 ci-ystals and appear to have crystallized at the same time with the feldspar. 

 These crystals are more recent than the rock in which they are embedded, 

 but they demonstrate that conditions necessary and sufficient to the forma- 

 tion of feldspar in the wet way have existed in tiiis region. The wdiole oc- 

 currence is such as to exclude the possibilit}' tliat these veins are of eruptive 

 origin. There is also abundant evidence of the presence of authigenetic 

 feldspar in tlie altered sandstones. 'J'he process usuallv cinumences in the 

 fine detritus which often composes tlie cement of the sandstones. Here 

 form slender, p<dysynthetic, plagioclase microlites, of such shapes and in 

 such grouping tiiat it is impossible to suppose them to ])e clastic constituent.s. 

 The larger grains are attacked later than the cement, aiul both quartz and 

 feldspar grains appear to be resolved into plagioclase, secoiulary quartz not 

 infrequently forming at the same time. Tlie corroded grains are often to 

 be seen surrounded by a fringe of authigenetic plagioclase microlites, the 

 nucleus remaining clear. The allothigenetic feldsjjar grains are also often 

 recrystallized without aiiv change in the external outline. In such cases an 

 aggregate results which is usually microcrystalline, but often ;dso includes 

 or may be almost entirely composed of lath-like, hemitropic lanielhe. 



Tliere appears clear evidence of the process by w-hicli tolcraljly large 

 plagioclases may be formed in the rocks which ha\e undergone metaso- 

 matic recrystallization. In some rocks which still retain an indubitably 

 clastic character, plagioclase microlites may be seen forming in groups of 

 almost identical orientation, but still separated by thin la}-ers of minerals 

 not belonging to tlie feldspar series. P]ven rough, crystalline outlines may 

 be traced surrounding such groups. The hemitropic lamella' in such cases 



