PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 419 



Sandstone. Conglomerate. Diabase.] 



Mi<: The section contains not only quartz but considerable devitrified glass. 

 The grains are not well rounded. The hematite coloring is evident. 

 One section. 

 Age. Potsdam. N. H. w. 



No. 556. SANDSTONE. (Red, shaly.j 



South shore of Siskiwit bay, Isle Royale. 

 Ref. Annual Report, x, page 50. 



Meg. A brick-red rock, fine grained, but holding small pebbles, all of which 

 are reddened and many of which are hard and siliceous. The rock effervesces slightly 

 in acid, showing the presence of some calcite. 



No section. 



Age. Potsdam. u. s. G. 



No. 557. CONGLOMERATE (with copper). 



"Cupriferous conglomerate, from the Island mine, Isle Royale, near the head of Siskiwit bay. This is also 

 argentiferous." 



Ref. Annual Report, x, page 50. 



Mry. A coarse, red conglomerate. The pebbles are usually pretty well rounded 

 and are of all sizes from very minute ones to those an inch or more in diameter; in fact 

 the whole rock is largely composed of these pebbles, there being comparatively little 

 siliceous cement. There is also calcite cement, and some native copper, although 

 the hand specimen does not show the latter clearly. The pebbles are all red and in 

 general can be said to be felsytes. Many of them are porphyritic with feldspars and 

 resemble devitrified glassy rocks. 



No section. 



Age. Potsdam; probably the Puckwunge conglomerate. 



Itcnturks. This rock is similarto the copper-bearing conglomerates on Keweenaw 

 point, and also to No. 155. TJ. s. G. 



No. 558. DIABASE (with olivine). 



Near the stamping mill of the Island Mine, Isle Royale. Near the creek coming through the location 

 [This mine is north from the head of Siskiwit bay, and perhaps 200 feet above the lake.] 

 Ref. Annual Report, x, pages 50, 51. 



Meg. Gray, with lustre-mottled spots, medium grained; not a fresh rock. 

 Mic. The rock is ophitic, and the olivines are altered to a greenish substance, 

 surrounded by much magnetite. 

 Two sections. 

 AIJC. Cabotian(?) N. H. w. 



