PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 705 



Greenwacke.] 



of the powder, and show the proportion of pyroxene in the rock. They give the 

 faintly green tint to the rock en ensemble. The sphene is not so much as to make any 

 impression on the powder nor on the rock as a whole. One section. 



Remark. This is a remarkable result. No such a rock could have been expected 

 in this greenstone. It is, of course, so far as it goes, testimony to the fragmental 

 character of the whole rock, for this is but a fragment like many fragments included 

 in it, though most of the foreign pieces are not of this character. The entirely 

 altered character of this rock leads one to expect that all the other fragmental parts 

 have suffered a similar profound alteration, or at least have suffered the same 

 metamorphosing force. 



I found a fragment of the anorthite which gave an oblique bisectrix n e . Extinc- 

 tion on it with a cleavage, was 29. While this is not determinative for anorthite, 

 it is not discordant with that mineral if the obliquity of the section be considered. 



I made an assay Boricky, and it gave chiefly lime, but also considerable soda, 

 indicating not a pure anorthite. 



(c) Meg. Is quite a different rock. It contains a large amount of quartz, so 

 much as to make this the most evident and abundant ingredient, and the rock can 

 be described best from that point of view. 



Mic. The quartz surrounds and encloses all the other substances except the 

 dark sub-opaque element, whose nature is problematical. The most frequent of 

 these enclosures is a triclinic feldspar whose extinction on n f in one of the larger 

 grains is 65, indicating andesine according to the late tables of M. Fouque (Bulletin 

 de la Societe de Mineralogie de France, 1894). This feldspar is sometimes in grains 

 of sizes sufficient for the determination of the orientation of the lamellee by the micro- 

 scope, when they are also well preserved, and such grains appear to have preceded 

 by but a very short interval, if at all, the small grains, which 'are completely 

 surrounded frequently by the quartz plages poikilitically, and which are much 

 changed to a mica, probably damourite according to Lacroix. These little changed 

 feldspars are frequently rounded, but were probably originally simple, stout crystals, 

 whose corners have been destroyed. Their habit and greater decomposition indicate 

 that they were originally of some other species, but it is impossible now to identify 

 it. It was probably more alkaline than the other larger crystals. 



The dark substance mentioned has the wandering outlines of a magma residue, 

 and its opacity agrees with that supposition. But it is not of uniform character. 

 Sometimes it surrounds the quartz and the undecayed feldspars. But throughout it 

 are shapes which indicate the former existence also of some of the changed feldspar 

 grains. They are now nearly opaque, but they pass by an insensible gradation into 

 the grains which are certainly of the more changeable feldspar. There is a grada- 



46 



