

MUSEUM OF COMPARATIVE ZOOLOGY. 



127 



case. The farther from tlic sandstone, and the nearer the heavy beds of 

 trap, the larger have been the deposits of copper, e. g. Chff, Central, and 

 Calumet and Heela. The conglomerate of this last mine was not the 

 home of the copper ; it is simply the place where it has been deposited 

 by secondary agencies. The statement now kept up for some thirty 

 years in Dr. Dana's works, that the copper is chiefly found at the junc- 

 tion of the sandstone and trap, is a good illustration of how an error once 

 m a text-book cannot be eradicated. 



Conclusions. 



The general geological structure of the region visited by ns is, then, 

 in general, as follows. Beginning on the southeastern side of Keweenaw 

 Point we find a sandstone and conglomerate overlaid by melaphyr. 

 This melaphyr is again overlaid by sandstones and conglomerates^ 

 principally the latter. The alternations of melaphyr, diabase, sandstone, 

 and conglomerate, with the melaphyr and diabase largely predominat- 

 ing, continue across the centre of the Point, forming its backbone. As 

 the northwestern side is approached, the sandstones and conglomerates 

 increase, while the melaphyr and diabase diminish, until a purely sand- 

 stone formation is reached. 



All these rocks taken together make one geological formation, and 

 have been laid down successively one upon the other, in order, going 

 from the east towards the west. These rocks arc known to form the 

 same series by their conformably overlying one anotlier. These traps 

 are old lava flows, spread out over the then existing sui'face along 

 a shore line. They have flowed the same as modern basaltic lavas do 

 imder like conditions, and retain the same characters, except* so far as 

 thoy have been modified by the agencies to which they have been sub- 

 jected since their outflow. They are known to be old lava flows by their 

 baking and indurating the immediately underlying rock, by their send- 

 ing dikes and tongues down into this rock, by tlieir scoriaceous charac- 

 ter on the upper surface, by their signs of having flowed, and by their 

 microscopic characters. That they were laid down before the over- 

 lying rock is shown by their producing no effect upon it ; by their 

 presenting on the upper surface only the irregularities and rounded 

 knobs that such surfaces are known to have, especially when they have 

 been worn ; by rounded pebbles and boulders of the underlying traps, 

 being enclosed in the overlying conglomerates ; by the absence of frag- 

 ments of the overlying rock iu the underlying one; and by the ab- 



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