138 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
Towa, and Minnesota,! Norwood has called them “ metamorphic shales,” in 
which he has since been followed by N. H. Winchell.? The possibility of 
a sedimentary origin for them is absolutely excluded by the completely 
crystalline interior texture, the highly vesicular character, the presence of 
unindividualized magma, the microscopic flowage structure, and the gradua- 
tion of each bed downward into vertically columnar, non-vesicular olivine- 
diabase.* 
The peculiar type of amygdaloid characterizing the so-called Ash- 
bed of Keweenaw Point is somewhat related to the last-described. It 
appears as a peculiar and irregular mixture of red sand and amygdaloidal 
material, and bears at first sight some resemblance to those conglomerates, 
already described,* in which the pebbles are amygdaloidal. But in the 
Ashbed the apparent pebbles appear on close observation to be mostly 
connected, and I am disposed to follow Wadsworth’ in considering that it 
represents a very scoriaceous and open layer, upon and within which more 
or less sand was subsequently deposited. 
In this connection it may be said that materials to which the term 
‘“‘ash” could be applied, in the sense of voleanic detrital material, are almost 
wholly wanting in the Lake Superior country. It is barely possible that 
the basic ingredient of the dark-gray sandstone and black shale of the belt 
above mentioned as extending from the Gratiot River, on Keweenaw Point, 
westward to Bad River, in Wisconsin, may be in part of this nature, as also 
the material of which the somewhat allied gray sandstone at Duluth is com- 
posed; but, with these very doubtful exceptions, I have met with nothing 
in the entire Keweenaw Series which could have originated as volcanic 
ash. I had supposed in the field that some of the crumbly, more or less 
obscurely stratiform beds of the North Shore might be of this nature, their 
weathered surfaces presenting at first sight the appearance of aggregated 
1Pages 345 to 361. In Norwood’s descriptions of the Minnesota coast this term constantly recurs, 
and the coast line is represented as chiefly formed by ‘‘ metamorphic shales,” which term, having ap- 
plied it first to the bedded amygdaloids, he spread to nearly all other amygdaloids, as well as to some 
of the more compact diabases. I could find no rock between Duluth and Grand Portage Bay to which 
the term would in any sense be applicable. 
2 Reports of the Geological Survey of Minnesota for 1878 and 1879, 
*See further as to these amygdaloids, Chapter VII. 
4Chapter II, p. 29. 
®Notes on the Iron and Copper Districts of Lake Superior, p. 112. 
