878 THE GEOLOGY OP MINNESOTA. 



[Granite. 



No. 2264. GRANITE. (Incipient. ) 



Near the same place as the last, but a little further south. 

 Ref. Annual Report, xxiv, page 80. 



Meg. A gray, f elsitic-looking rock, without apparent phenocrysts, except pyrite, 

 which is plainly visible in cubic forms; has a similar dike-like action, cutting a 

 greenstone conglomerate. 



Mic. The size of the grains in the composition of this rock is about the same 

 as that of No. 2263, and the materials are about the same. The hornblendes, how- 

 ever, are more conspicuous, and in some cases they show their derivation from augite 

 by the peripheral growths beyond the augitic nucleus, the whole being now converted 

 to hornblende, but having a central area of greater absorptive power. For the most 

 part, however, the hornblendes seem to be independent of any augitic nuclei, and 

 especially the smaller ones. These degenerate in size to mere club-shaped spicules 

 in which form they are very abundant in some places, and these spicules still further 

 degenerate in size, becoming only minute globules, illustrating the infantile growths 

 which have been noted in the case of quartz, feldspar, siderite and pyroxene and 

 other minerals. 



The principal ingredient of the rock is feldspar, but it is so far obscured by decay 

 that it cannot be specifically identified, except that, in some cases, it can be seen to 

 be of a striated species. The grains are, it is true, held together by a background of 

 similar materials, through which has run a secondary feldspathic crystallization, and 

 this has also permeated the old feldspars, but the secondary growths are very incon- 

 spicuous, and do not generally appear about the margins of the old feldspars. Occa- 

 sionally, however, an old feldspar can be seen to be wholly rewrought by a more or 

 less distinct granulitization, and the hornblende spicules run into such grains. 

 Amongst the original clastic debris was an occasional distinct quartz grain. Such 

 grains are somewhat enlarged by secondary growths, and finer secondary quartz 

 appears also where there is no evidence of any original quartz. Scattered through- 

 out the slide, and especially in the old feldspars, is a finely granular and highly 

 refractive mineral which cannot be certainly determined, but may be epidote. With 

 a little leucoxene and some Muscovite this list completes the mineral composition. 



Age. Intrusive in the Upper Keewatin. 



Remark. This rock is intermediate, in crystalline state, between the last (No. 

 2263) and a compacted graywacke. The decay of the feldspars is exactly like that 

 of the graywackes, and is here supposed to have taken place prior to consolidation 

 into a rock, at least before the metamorphism. The mass, when still in its clastic 

 condition, was under such pressure that it was forced as an intrusive amongst the 

 adjoining rocks, and this generated an incipient recrystallization which was arrested 

 before it completely permeated and altered the clastic grains, hornblende and mus- 

 covite being the first and most obtrusive forms in this regeneration. . N. H. w. 



