MUSEUM OF COMrAKATIVE ZOOLOGY. 



123 



The Veins and Copper Deposits. 



At some time after the sandstones, conglomGratcs, and traps were 

 laid down, extensive and deep-seated fissures were formed, generally 

 extending, in the distriet north of Portage Lake, aerosa the beds. These 

 fissures seem to have been formed by powerful movements of diflerent 

 parts of the roeks, causing their cross fracture and dislocation. The 

 movements have been repeated from time to time, causing a rubbin'^- 

 grinding, breaking, and polishing of the parts adjacent to the fissures, 

 forming numerous slickensides. These movements would not cause ir- 

 regular and secondary fracturing to any great extent after the main fis- 

 sures had been formed, in the heavy-bedded diabases or the sandstones • 

 but in the seoriaceous and thin-bedded melaphyrs, the tendency to yield 

 to the pressure was much greater, and the rock adjacent to the fissures 

 was broken up to a considerable extent. During the time of the fractur- 

 ing, and since, the fissures served as channels for water. In the seori- 

 aceous and easily decomposable melaphyrs the water served by its 

 decomposition of the adjacent rock to widen the fissures, but in the 

 diabases, although their composition was the same, their structure and 

 the absence of much glass prevented the same results occurring. The 

 sandstones and conglomerates, from their being principally composed 

 of trachytic and rhyolitic material, and from their structure, suffered 

 little, compared with the melaphyrs, from vein formation. 



In many localities the evidence is strong that the percolating waters 

 were hot- in others, as remarked by Marvine, no sign exists that it was 

 above the temperature of the present day. AVater penetrated with 

 greater or less readiness through the traps themselves, causing their de- 

 cay and alteration, while the substances taken up in solution by it were 

 deposited in the fissures, cells, and other open spaces in the rock. The 

 filling of pre-existing cavities and fissures gave rise to the amygdaloidal 

 structure of the traps and the vein material ; the decomposition of the 

 trap, and its replacement of some materials by others, gave rise to 

 the pseudo-amygdaloidal structure. This structure exists not only in 

 the non-scoriaceous or non-cellular portions, but also in the truly seori- 

 aceous or cellular portions. 



All or nearly all of the material filling the veins and adjacent traps 

 appears to have been derived from the decomposition of the traps them- 

 selves, and not brought in from extraneous sources, — this decomposition 

 being brought about by the medium of the percolating waters, whoso 

 course seems, in many cases at least, to have ^ccn downwards. The par- 



