PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 263 



Basalt.] 



ophitic structure, but occasionally they are seen to interfere with the outlines of the 

 small augites. 



The augite, though in small grains, can easily be distinguished. Its grains are 

 larger than the olivine grains, and they have a yellowish color, and less refractive 

 power than the olivines. They are comparatively scarce. 



Olivine is in very small grains, scattered abundantly throughout the slide, some- 

 times embraced in the augites and sometimes in the feldspars. It is well preserved 

 considering the minuteness of the grains. 



Glass is less common than in No. 176 or in No. 217, but still occurs sufficiently 

 to warrant the designation basalt. It surrounds all the other crystals. It is not so 

 much ferrated as in No. 217. 



Occasional magnetite, even having its rectangular outline, is also to be observed. 



One section. 



Age. Cabotian. N. H. w. 



No. 219. BASALT. 



" A little beyond [east of] the last locality [a point near the Brule river, off which lies the principal islands], 

 and just as the rock disappears again, it suddenly becomes slaty or closely jointed and laminated, dipping S. 10 

 E., and more enduring. In this condition it forms some of the islands near the beach, and also rises fifty or 

 more feet near the coast back from the water." 



Ref. Annual Report, ix, pages 55, 66. 



Meg. A very fine-grained, hard, compact, brown rock. It is of the same general 

 character as Nos. 216 and 217. 



Mic. Microliths of plagioclase are the most noticeable feature of the section, 

 but between these, and not sharply separated from them in size and shape, are other 

 plagioclases; thus nearly the whole section is composed of feldspar. There are also 

 small, more or less angular grains of augite, also magnetite, and some alteration 

 products. In structure and composition the rock resembles No. 218. 



One poor section. 



Age. Cabotian. u. s. G. 



Remark. Nos. 217, 218 and 219 seem to be a repetition of Nos. 213, 214 and 215 

 outwardly, but the latter trio is more acid. It may be that structurally they are 

 nearly related. No. 176 belongs with Nos. 217, 218 and 219. 



These beds (Nos. 217-219) seem to have been disturbed by some upheaval, 

 and appear in nearly all respects like those of No. 208 (fine grained), though not so 

 conspicuously exposed. N. H. w. 



No. 220. BASALT. 



Mouth of Brule" river, forming a dull point. 



Ref. Annual Report, ix, page 56; Annual Report, x, page 43. 



Meg. A dark -gray, diabasic rock, of fine grain. It contains numerous small black 

 blotches. (Compare No. 539.) 



