208 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
layers previously concealed aids in increasing this width, which on the 
west line of range 41 is as much as five miles, as against only three on the 
Ontonagon. : 
This increase in surface width becomes very rapid in the next ranges 
westward, the width in the east part of range 42 being as much as eight 
miles. Beyond range 44 the Eastern Sandstone and the depression it oc- 
cupies come to an end, the South and Main Ranges coming together. As 
already explained, the South Range is made up of belts of the usual basic 
eruptive rocks dipping northward and constituting the lowermost part of the 
Keweenaw Series; their separation from the beds of the Main Range is 
due to faulting. It has also been shown already that this fault dies out to 
the westward, the South and Main Ranges coming together. As a result of 
this the width of country occupied by the Lower Division of the series 
becomes on the Black and Presqu’ Isle rivers as much as ten miles. 
East of the Ontonagon River the course of the rock belts is southwest, 
or even a little south of southwest. After crossing the Ontonagon there is a 
change to a direction only slightly south of west, and for the rest of the dis- 
tance to the Montreal the general trend of the belts constituting most of the 
Lower Division oscillates between 5° and 20° south from west. On the 
Montreal a rapid change takes place to a direction only 30° to 40° west of 
south. With regard to these lower belts in this distance I have nothing 
new to offer, and can only say that so far as known they present the usual 
diabases, melaphyrs and amygdaloids, with some porphyries, and in the 
lowest portions coarser rocks belonging to both the olivine- and orthoclase- 
gabbros. 
The upper belts of the series and all of the Upper Division are not in- 
cluded in the statement just made as to general trend. On following them 
westward they are found bending out to the north in a curious loop, return- 
ing on the western side of the loop to their original positions. The neck of 
the loop trends nearly north, or at right angles to the Main Range, but near 
the lake its end bends well around to the eastward. Within this loop are 
the Porcupine Mountains, whose interior portions are made up of felsitic 
and quartziferous porphyries, without any definitely traceable structure. 
These porphyries plainly enough belong to the same belt or belts which 
