PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 415 



Conglomerate.] 



Remark. This ingredient of volcanic glass in this rock, indicates that it is not 

 probably at nor near the base of the Animikie, for no such rock could be obtained in 

 such quantities from the Archean. That had been completely crystallized before 

 the opening of the Animikie. This rock, therefore, and the formation to which it 

 belongs, carrying with it the underlying basal conglomerate, seems to be later than 

 some Cabotian eruptives, and hence is probably nearer the bottom of the Potsdam. 

 The only explanation of a possibly Animikie age would be the presumption of 

 cotemporary volcanic action in early Animikie time by which such materials could 

 be incorporated; but that presumption again is just as reasonable at the time of the 

 earlier part of the Potsdam, and is negatived by the red rock material found in the 

 underlying conglomerate which is a product of the Cabotian disturbance. N. H. w. 



No. 549. CONGLOMERATE. (Breccia?) 



Grand Portage island. The cementing rock is a (iiiartzyte or grit like No. 548, but coarser. Sixteen feet 

 in thickness. 



Ref. Annual Report, x, pages 46, 47. 



Meg. The rock is a gray conglomerate, of rather coarse pebbles. The prepon- 

 derating element is clear quartz, or gray quartzyte. In the case of the gray quartzyte 

 the pieces are flat, slaty and firm, and can be referred directly to the quartzyte slates 

 of the Animikie, which rises in the adjacent hills of Grand Portage to the hight of 

 three or four hundred feet above the level of the lake. The rest of this conglomerate 

 consists largely of finer debris of the same kind, but also contains numerous pebbles 

 of red quartz-porphyry, red felsyte and red granite. This can also all be referred to 

 the hills at Grand Portage. 



Mic. The thin section not only confirms the megascopic appearance, but adds 

 some interesting other facts, viz.: There is a considerable ingredient of originally 

 volcanic material amongst the debris. This is in the form not only of devitrified 

 glass grains, but of porphyry pebbles in which the feldspar crystals are still apparent 

 by their forms, although their optical properties are lost by a profound change 

 that has affected the conglomerate. In short, the conglomerate seems to have been 

 formed subsequent to the origination, in that neighborhood, of acid lava flows and 

 quartz-porphyries. 



Another fact which is vividly revealed by the thin section is the calcification of 

 the rock, and of all the elements of which it is composed except the quartz. The 

 pebbles of igneous rock, and the porphyritic feldspars contained in them, are so 

 permeated by calcite and so changed to kaolinic particles that they cannot be 

 identified except by their structure and relations to surrounding portions. The 

 feldspars do not extinguish, but present ever a lightness which is flecked with light 

 and dark particles which under rotation shift and disappear and return again, making 

 a constantly varying fine mosaic. Two sections. 



