PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 273 



Basalt.] 



containing foreign material, i. e., a grain of olivine. It is possible that other pheno- 

 crysts would in thin section show this feature more prominently. The groundmass 

 is composed of playioclase, augite, olivine and magnetite as original constituents. The 

 rock, as a whole, has been considerably altered and fissured; the fissures are filled 

 with hematite and a greenish yellow serpentine-like material, and these two substances, 

 especially the latter, are developed throughout the rock, replacing the olivines and 

 most of the augites, and also filling spaces which may possibly have been occupied 

 by a glassy residuum; a little secondary quartz is also present. The rock is inclined 

 to a hypidiomorphic structure, rather than to the ophitic, but, as the augites are so 

 far gone, the relation of this mineral to the feldspar is not clearly made out. The 

 feldspars, however, while usually in more or less idiomorphic grains, are frequently 

 partly or wholly allotriomorphic. 



Three sections. 



Aye. Mauitou(?) u. s. G. 



No. 235. BASALT (?) 



"A rock similar to No. 232, cut by the dikes, having a slaty structure without any dikes; forms the beach 

 next north of the dike No. 234, which is out in the water." 



Ref, Annual Report, ix, pages 59, 60; Annual Report, x, page 47. 



Meg. A compact, hard, very fine-grained, almost aphanitic, red-brown rock. 



Mic. The section, which is a very poor one, shows microliths of feldspar and 

 grains of magnetite, in a reddened background whose nature is not discernible on 

 account of the thickness of the section. There is also one sub-angular quartz grain 

 present. 



One section. 



Age. Cabotian. 



Remarks. This rock is probably the same as No. 232. u. s. G. 



No. 236. BASALT. 



" Prom a dike twenty-one feet wide; a fine-grained, black basalt, running out into the lake about 250 feet, 

 but often in the form of islands that occur a little out of line. The basaltic structure of this is very irregular. 

 In some places it is fine and in others it is coarse; runs N. 15 W., being intersected by the dike No. 234, appar- 

 ently in the same manner as No. 233." 



Ref. Annual Report, ix, page 59. 



Meg. A heavy, compact, very fine-grained, greenish, black rock. It contains a 

 little pyrite. 



Mic. The section shows microliths of plagioclase in a confused, altered back- 

 ground, which is composed of magnetite, chlorite and cloudy, greenish, or grayish 

 (sometimes with a shade of brownish) areas. These cloudy areas under polarized 

 light, are seen to be sometimes feldspathic, but most commonly show no effect on polar- 

 ized light, except for a few bright points. The whole background of the rock was 



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