MUSEUM OF COMPAUATIVE ZOOLOGY. 



95 



copper : '' The presence of copper in the sandstones suggests another 

 origin — namely, that it may have originally been deposited with the 

 quartz-ore sediment as a finely divided sulphide from sea-water under 

 the influence of organic matter, and by subsequent oxidation and solu- 

 tion have been removed and collected in the rocks below The size 



of the accumulated masses of metal appeal's to be mainly dependent 

 upon the size of the cavities in which they are deposited, whether in 

 the amygdaloids or in the main fissures ; and their absence in the com- 

 pact traps is probably only due to the non-occurrence of such cavities. 

 In almost all cases the introduction of the metal has been preceded 

 by the deposit of minerals produced from the decomposition of the 

 rock, such as quartz, calcite, cldorite, and zeolite ; and in the larger 

 cavities it is often followed by transparent crystals of calcite, which are 

 fonned over branching masses of copper, or even show signs of simulta- 

 neous deposition, being filled wuth fire-spangles of metal arranged par- 

 allel to the diagonal striations or lines of growth on the rhombohedra. 

 Similar alternation>s in the formation of zeolites, more particularly 

 analcime, have been described by Whitney." He is inclined to regard 

 the deposition of the copper in the amygdaloids as having tidcen place 

 prior to the filling of the veins, the former serving as feeders to the 

 latter. (I. c, pp. 4G0, 4G1.) 



In a paper read before the Boston Society of Natural History, June 

 5, 18G7,* Mr. Alexander Agassiz remarks: "Foster and Whitney, in 

 their Report of the Lake Superior mineral district, represent the sand- 

 stone on the south side of the trap range of Keweenaw Point as dip- 

 ping south and resting conformably upon the beds of trap of the north 

 side of the anticlinal axis of Keweenaw Point. Tliis anticlinal axis 

 formed by the Bohemian Mountain, as asserted by Foster and Whit- 

 ney, is not found further south as far as I have had occasion to examine. 

 In two of the ravines cut through the sandstone by creeks flowing in 

 an easterly direction from the crest of the range towards Torch River, 

 near the head of Torch Lake, we find good exposures of the sandstone 

 resting unconformably upon the trap which has still the same northern 

 dip as farther west, of about 42°. The sandstone within a distance 

 of one hundred feet from the trap, dipping nortli 42°, lies horizontally, 

 or rather has at the outside an inclination of 1-|° or 2° south." At the 

 falls of the Douglass Houghton Creek, he says : " The creek winds its 

 way through a deep ravine cut out of the sandstone, and at the junction 

 of the sandstone and trap, falls a depth of one hundred and sevcnty- 



* rroooedings, XI. 244 -2i7. 



