PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 283 



(iray Sandstone.] 



also many small amygdules of calcite and chlorite. The peculiar feature of the rock 

 is the large amount of calcite; this mineral not only fills the arnygdaloidal cavities 

 and the spaces left by the porphyritic crystals, but exists in large amount in the 

 groundmass of the rock. The calcite evidently makes up more than one-half of the 

 rock mass. 



Two other sections show essentially the same features described above, with the 

 additional fact that the rock has included fragments and grains of the adjacent 

 quartzyte. Sometimes rounded grains of quartz are seen in the midst of the basaltic 

 mass. 



Three sections. 



A<ji>. Manitou. u. s. G. 



Ecinarlc. Whether this rock be an amygdaloid or a tuff or both (more probably), 

 it is a surface rock, and not an intrusive; and as it is overlain by a sandstone it indi- 

 cates an interrupted volcanic action in the near vicinity during the accumulation of 

 the fragmental rocks at the base of this island, and it therefore makes more plausible 

 the suggestions of glass in the conglomerate No. 254. N. H. w. 



No. 256. GRAY SANDSTONE. 



"Sandstone; of even grain and bedding, lying between layers of trap-rock immediately over No. 255." 

 Grand Portage island. 



Kef. Annual Report, ix, page 61. 



A light-gray, fine-grained sandstone or quartzyte. It contains a few 

 small red grains feldspar. The hand sample shows a few dark-gray, or greenish- 

 gray, spots roughly circular in outline and about a quarter of an inch in diameter. 

 This spotted character is somewhat similar to the spotted rocks of the Animikie on 

 Pigeon point (see description of future rock numbers, and also W. S. Bayley, Bulletin 

 cix, U. S. Geol. Survey). 



. The section is composed almost entirely of interlocking, angular grains of 

 A few semi-rounded grains are seen, but the most of them show no evidence 

 of having been rounded. Practically no cementing material is present. The various 

 grains fit into each other, and some of them by their outlines show that they could 

 not have had this form as original fragmental grains. It seems evident that they 

 have been enlarged by the addition of secondary silica, but no distinct examples of 

 this were seen. There are also a few grains of feldspar o>1lt<>rlxc, inii-rncliiii: and 

 playioclase and some areas composed of very minute grains, probably of quartz. 



One section. 



Aye. Potsdam. 



Remarks. The rock has been changed somewhat from its original fragmental 

 state and would, perhaps, better be styled a quartzyte than a sandstone. The change 



