150 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
sandstone, but I have been able to detect no trace of fragmental origin, the 
rock under the microscope, as well as on the large scale, presenting all the 
characters of an original porphyry. The Mount Houghton porphyry seems 
to constitute a belt interstratified with the prevailing diabases and standing 
at a very high angle. It has a considerable lateral extent, and I am dis- 
posed to place with it the quartzose porphyry of the Torch Lake Railroad, 
south of the Calumet mine, which is evidently the source of the pebbles of 
the Calumet conglomerate. 
In the region between the Ontonagon River of Michigan, and the Bad 
River of Wisconsin, true massive quartziferous porphyries are largely de- 
veloped. The exposures appear to lie in certain horizons, at least two of 
which have been recognized, and so to constitute layers in the series, though 
evidently very much less regular ones than those of the diabase. 
In the Porcupine Mountains:a quartzose porphyry constitutes the cen- 
tral mass of the mountains, the beds of sandstone, conglomerate and slate 
dipping away from it on all sides save where it connects on the south with 
the Main Trap Range. Here the subordinate features of the porphyries of 
the North Shore are seen constantly repeated, but the mass as a whole is 
without any trace of stratification. The structure of these mountains is 
described and illustrated in some detail in a subsequent part of this report. 
As already indicated, I conclude with regard to all of these porphyries 
that they are unquestionably of eruptive origin. That they were formed 
both after and before the more common basic eruptive rocks is shown by 
such occurrences as that of the Great Palisades of the North Shore. These 
porphyry eruptions were evidently more plenty in the earlier part of the 
time of formation of the series. Still, in the Ontonagon region they occur 
at quite a high horizon. ‘The structure of the Porcupine Mountains has so 
much in common with that of the laccolitic mountains of Southern Utah, 
described by G. K. Gilbert, that they might be supposed to owe their exist- 
ence to an eruption of the porphyry of their central portions, in which 
case we should have to believe in an eruption of acid rocks at a time quite 
subsequent to that of the formation of the latest of the basic flows. But, 
as shown subsequently, these mountains owe their existence in all prob- 
