﻿126 H. A. Wheeler — Temperature Observations 



cellent opportunity for obtaining data as to the rate of thermal 

 increase with descent into the earth. While the usual thermic 

 gradient is between 50 to 55 feet for an increase in tempera- 

 ture of 1° F., exceptional gradients, both much higher as well 

 as lower than this, have been obtained in certain localities. It 

 was to add the evidence of this region in respect to this phe- 

 nomenon, which is of economic as well as geological import- 

 ance, that the writer made the following observations in some 

 of the deeper mines of this now quite old mining district. 



Keweenaw Point is a tapering peninsula that boldly extends 

 in a general northeasterly direction out into the middle of Lake 

 Superior, from its southern shore, for a distance of some 70 

 miles. It essentially consists of a series of parallel belts of 

 fragmental and plutonic rocks that abut against sandstone, of 

 supposed Potsdam age, on the eastern side, while they are con- 

 formably overlaid by sandstones and shales of Lower Silurian 

 age on the western side. The plutonic rocks consist of amyg- 

 daloidal and compact melaphyres and diabase, while the inter- 

 calated fragmental rocks comprise numerous belts of a red 

 conglomerate, the pebbles of which consist largely of quartz 

 porphyry. These conformable belts have a dip of about 55° 

 with a nearly northerly strike at the southern end, and gradu- 

 ally become flatter, and veer around until, at the extremity of 

 the Point, at their northern end, the dip is only 20° and the 

 strike easterly. The fractures incident to such a bending of 

 the formation has resulted in fissures, in which some of the 

 older mines are located, that are filled with zeolitic and 

 other mineral matter and often are richly charged with copper. 



Most of the mines are in either the melaphyre or conglom- 

 erate belts, in which the copper occurs disseminated in grains 

 and masses in the native condition. The whole area is more or 

 less thickly overlaid with glacial drift which in some cases is 

 over 100 feet thick. The temperature observations were made 

 during a rapid trip and the depths noted were taken from the 

 mines, maps and other data obtained on the premises. Special 

 care was taken to avoid modifying conditions at the observing 

 stations, such as draughts, water, working places, etc., while 

 duplicate readings were made of the thermometer. While the 

 results obtained can scarcely claim to be more than close 

 approximations, and are liable to be modified by more accurate 

 and extended observations, they clearly show that the thermic 

 gradient in this region is one of the lowest that has ever been 

 noted ; and the variations in the different mine seems to indicate 

 the cause for this very low rate. The results of the observa- 

 tions are given in the following table : 



