PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 435 



Amygdaloid. Copper and silver.] 



believe that both Nos. 578 and 579 are parts of a general basal conglomerate into 

 which the copper was introduced at some later date. 



No section. 



Age. Cabotian(?) (Perhaps in the basal, or Puckwunge, conglomerate of the 

 Potsdam). N. H. w. 



No. 580. AMYGDALOID. 



Minong mine, Isle Royale. Adjoins the copper-bearing rock. 

 Ref. Annual Report, x, page 54. 



Meg. Coarse, green amygdaloid, the cavities being filled with chlorite and 

 geodic quartz, coated with green, sometimes also with calcite and laumontite. The 

 amygdules make up more than one-half the bulk of the whole. 



Mic. The rock is very fine grained, and was apparently originally in part glassy, 

 the crystalline condition being due in part to devitrification. The chloritic rosettes 

 give the black cross of spheruliths. Owing to the thickness of the only available 

 sections no careful study is possible. 



Three sections. 



Age. Cabotian(?) 



Remark. The fresh aspect of this loose amygdaloid resembles some of the 

 Cabotian surface eruptives already described on the lake Superior shore in the vicinity 

 of Duluth and eastward. So far as can be judged from the hand samples collected 

 of Nos. 578, 579 and 580, they are from surface igneous rock, and appear to belong 

 in the Cabotian red-rock and surface lavas. They would thus represent the south- 

 ward flows from some great Cabotian dikes that occurred further north. The writer 

 did not make sufficient examination to warrant him in holding a positive opinion as 

 to the age of this copper-bearing rock. N. H. w. 



No. 581. COPPER. 



Minong mine, Isle Royale. 



Ref. Annual Report, x, page 54. 



Spreading, frond-like, crystalline, partially coated with malachite and cuprite, 

 and bearing a little calcite. N. H. w. 



No. 582. COPPER AND SILVER. 



Minong mine, Isle Royale. 



Ref. Annual Report, x, page 54. 



A small slab, or scale, about an eighth of an inch in thickness, and about two 

 inches in length by an inch in width, but of irregular shape, consists principally of 

 copper, but there are eight separate masses of silver lying in the copper. These 

 small masses vary from the size of a pin-head to that of a field-bean. They are 

 roughened, like the copper, by reason of deposition on some rough surface. There 



