PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 209 



Aporhyolyte.] 



than the former, and is embraced by it. The small rounded pyroxenes are some- 

 times sprinkled through a single feldspar to the number of four or five independently 

 oriented grains, but in some cases it appears that large pyroxenes were corroded to 

 a mere skeleton, and were then embraced by the feldspars. Thus the orientation of 

 many isolated small pyroxenes is the same. That they once were united in a single 

 crystal is proved by the fact that they are often still connected in a series with nar- 

 rower and narrower links between them till finally the link is entirely lost and only 

 the common orientation remains to show their former continuity. When the pyrox- 

 enes are of different orientation it is evident that the corrosion went so far as to 

 break up the crystal and disturb the relative positions of the parts. 



These small, rounded pyroxenes, originating in this manner, suggest a possible 

 cause of the "granulitic" phase of the gabbro. This phase is seen in its fully 

 developed state, in Nos. 122 and 131, both of which are parts of the same diabase 

 sheet as the basic rock, No. 137.* 



The green substance, as stated, is probably derived from an alteration of the 

 non-differentiated portion of the basic magma, while the red may have resulted from 

 the inclusion of portions of the Cabotian rhyolyte. 



Other sections, made by Marchand, afford some further data as to the nature of 

 the pyroxene. When sections are favorably cut, /. e., perpendicular to the prism, 

 there are seen three coarse cleavages, one being parallel to the direction of the optic 

 plane, and hence parallel to the side 010, which is a character distinguishing the 

 cleavage of ilityix/tfe from the fine cleavage of diallage, the latter being perpendicular 

 to the optic plane in such a section. The aspect otherwise of this pyroxene is that 

 of augite. 



The sections also contain considerable apatite, some sphene, some biotite, and in 

 the red portion of the rock, much heiiHiflfc. 



Four sections. 

 Aye. Cabotian; Beaver Bay diabase. N. H. w. 



No. 138. APORHYOLYTE. 



Top of the Great Palisades, 315 feet above the lake. 



Ref. Annual Report, ix, page 35; Annual Report, x, pages 38, 141; Proceedings American Association for 

 the Advancement of Science, vol. xxx, page 164. 



Meg. All aphanitic brownish gray rock, with porphyritic crystals of quartz 

 and of whitish decayed feldspar. 



Mir. The groundmass of the rock under polarized light is of micropoikilitic 

 quartz areas inclosing the other materials. The groundmass is similar to No. 68. 

 Phenocrysts of t/iimi:, with their angles more or less rounded, are rather common 



* Compare W. S. BAYI.EV. The peripheral phases of the great gabbro mass of northeastern Minnesota. Journal of 

 Geology, vol. ii, page 814. 



15 



