22 Dr. 31. E. Wadsicorth — Copper Deposits in Michigan. 



30° to 60°, and also cut across by fissures running north-west to 

 south-east, and you have a fair idea of what has happened. 



It is well known that in all regions where volcanic forces have 

 been active, that when these forces die out, hot-water action is 

 one of the last results, the waters gradually growing cooler until 

 they are at the noi'mal temperature, so that in time there is no 

 evidence of the former hot state except that shown by its results 

 on the rocks. 



In the Lake Superior district the water action — mostly hot, 

 sometimes cold — was strongly marked during the fissuring and 

 movement, or, as it is technically termed, " faulting" of the rocks, 

 as well as for a long time subsequently. During the time of this 

 water action all the rocks, without exception, were penetrated by 

 these percolating waters, much of their materials dissolved out, 

 or chemically re-arranged, or removed and replaced by other 

 elements. It was then that the native copper now found in the 

 rocks was stored up on its present banks of deposits, from which 

 it is now being rifled by means of the drill, sledge, and dynamite. 

 Three different systems of local deposit have been employed by 

 Dame Nature on Keweenaw Point. The profound and repeated 

 fissuring previously spoken of caused huge vaults to be made where 

 the percolating waters left, securely locked up, their treasures of 

 copper ; and here the largest single deposits were made, and the 

 drafts have been fully honoured. The vaults extend mainly in 

 a north-west and south-east direction, cutting across the country. 

 These deposits are technically known as fissure veins ; and as 

 examples there may be cited the Central, Cliff, Phoenix, and other 

 mines, mainly on the northern end of Keweenaw Point. 



As one would naturally suppose, the various lava-flows would 

 differ in thickness, owing to the varying amounts of volcanic 

 material erupted, as well as to the inequalities of surface. Like 

 variation would also exist in the extent and amount of deposited 

 conglomerate, shale, and sandstone, on account of similar inequalities 

 of the surface during its formation, and the area exposed to the tidal 

 or wave action. 



Eeturning to the lava-flows, it has been found that the thinner 

 ones are more glassy, and hence more easily acted upon by the 

 percolating waters ; thus, large amounts of the original rock materials 

 have been dissolved out, removed, and their places, as well as those 

 of all other cavities, have been filled with deposits of copper and 

 other mineral matter. These deposits are mined and form the 

 melaphyre (locally called amygdaloid) mines, such as the Quincy, 

 Osceola, Franklin, Atlantic, and Huron. These mines are worked 

 on old lava-flows that have been impregnated with copper, the same 

 as a flow from Mount Etna might be worked, if it were likewise 

 filled with valuable mineral. Although it is usual to speak of these 

 mines as worked upon veins this is an error, as the}' have neither 

 the structure of a vein, nor any sign of a vein about them. They 

 are simply flow deposits. 



At the same time our fissure, or vein, and overflow deposits were 



