SOME PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 267 



temperature with depth is found to be very small, about 1° in 103 

 feet. Lane suggests that this low temperature gradient may be due 

 to chemical reactions which absorb heat, such as the deposition of 

 copper; the diffusivity of the strata, permitting the early and free 

 escape of heat; downward absorption of water carrying with them 

 cooler temperatures of the surface; recent deposition of surface 

 drift, and the relative exhaustion of the internal supply of heat by 

 the Keweenaw and earlier eruptions. 



The mine waters of the region are of three types: (i) normal 

 surface carbonate waters, in which calcium and magnesium are in 

 excess of sodium; (2) waters of intermediate levels, 1,000-2,000 

 feet, high in sodium and chlorine, in which lime and magnesium are 

 subordinate; (3) waters of deep levels in which lime chloride domi- 

 nates, and in which magnesium is absent or nearly absent and 

 sodium subordinate. Waters of type (2) are associated with the 

 richest part of the lodes and with the occurrence of silver. The 

 deep waters are regarded by Lane as connate waters, but not nec- 

 essarily derived from the ocean of that time. 



Lane regards the lavas and sediments as the original source of 

 the copper. All the lavas and sediments contain at least o . 02 per 

 cent or more of primary copper. The lavas and sediments he 

 believes were deposited rapidly one upon the other, the lavas 

 retaining their heat for a long period of time. Water and gases 

 containing chlorine were given off by these lavas and these caused 

 a hydration of the silicates of the rock, developing chlorite and 

 epidote and some native copper. This first alteration, however, he 

 believes was insufficient to account for the development of all the 

 copper, since the occurrence of the copper in the porous parts of the 

 formation indicates that circulation of water must have taken place. 

 The circulation which Lane believes developed the ores was set up 

 largely through the cooling of the rocks. As the rocks cooled they 

 contracted and afforded openings for water which was sucked in 

 from the surface. A large part of this water entered into chemical 

 combinations with the rocks and thus a very strong and long- 

 continued downward circulation must have been set up, since the 

 greater portion of the rocks have been hydrated to chlorite. This 

 reaction also involved solution of sodium silicate, copper, and lime 



