20 Dr. M. E. Wadsworth — Copper Deposits In Michigan. 



are due to the contraction of the quartz and the direct strain thus 

 brought to hear on the surrounding glass, we cannot but notice that 

 these cracks undergo no change in direction or character when 

 traced into the glass. So much is this the case that I think it 

 would be very difficult for anybody to tell from the plate, the 

 drawing, or the photograph, even indeed from the examination of 

 the slide itself except in polarized light, where the quartz ends and 

 the glass hegins, so exactly alike are the cracks in the two sub- 

 stances ; for this reason a small drawing (fig. la) was added to 

 plate xviii. 



There seems to me to be no reason why the two types of curved 

 cracks which mainly characterize the quartz and matrix respectively 

 should not be regarded as distinct, and even receive separate names ; 

 but it will have to be borne in mind that the quartz types can be 

 artificially produced in structureless balsam, that they extend from 

 quartz to matrix, and that they must undoubtedly occur inde- 

 pendently in glass ; and also that, on the other hand, the perlites of 

 the type more characteristic of glass are often completed in quartz 

 with little or no change in character. The main fact still remains 

 untouched, that rectilinear and spherical contraction cracks indiffer- 

 ently traverse two distinct constituents of the rock, and that the 

 ' little rift ' is not solely the fault of either of the ' contracting 

 parties.' 



IV. — The Copper Deposits of Michigan. 1 



By M. E. "Wadsworth, Ph.D., 

 Director of the Michigan Mining School. 



IN looking at the map of the Great Lake region, you have all 

 noticed the backward bending thumb of Michigan projecting 

 into the icy waters of Lake Superior ; yet but few of you, perhaps, 

 have realized that extending along that thumb there runs a band 

 or ring of native copper. It does not, like most gold or silver bands, 

 extend around the finger, but along it — from the base of the hand to 

 the end of the thumb — this central band lies embedded in the flesh 

 binding all together. Shall we now dissect it, laying bare its flesh, 

 muscle, and bone, and try to explain its marvellous organization ? 



To do this, it will be necessary to drop much of our simile and to 

 make as clear as possible the geological structure of the district in 

 question. Eoughly, its central portion, extending from the south- 

 west to the north-east, may be said to be made up of an elevated 

 plateau, bearing upon its wrinkled surface protuberances or hills — 

 locally called mountains — like warts upon a finger. Flanking both 

 sides of this higher land lie lower lands extending down to the level 

 of the lake. This lower level is formed of hardened beach muds, 

 sand, and shingle, laid down on the shores of a tide- washed sea. 

 We find in it the ripple marks made by the waves, the mud cracks 

 formed when exposed to the drying sun, and the prints of the soft 



1 Read at the Annual Convention of the Michigan Bankers' Association, 

 September 12, 1895. 





