USGS1885
Although the copper-bearing rocks of Lake Superior and the adjoining formations have attracted the attention of geologists for fifty years past, there are yet remaining unsolved very many problems with regard to them. We have ourselves, indeed, written at some length with regard to these rocks, and one of us has even attempted a general account of the series as a whole. In the course of our investigations each of us has been obliged to turn away from very many tempting problems in structure and genesis, where there was every promise of successful solutions if only the element of time had not been lacking. Not the least important among the problems which we have hitherto been thus obliged to pass by is that of the exact nature of the structural details at the contact line between the Keweenaw Series and the so-called Eastern Sandstone. This contact line one of us had already examined sufficiently to satisfy himself of the general relations of the two formations concerned, but he had not been able to go further than this. Recently, however, the opportunity has offered for us to make together some additional observations on this line of contact, with results that have proved to be of so conclusive a nature that we think it well to publish them in the form of a bulletin. In what follows the several places at which this junction has been studied by us are taken up in order from east to west. Preceding our own descriptions of each one of these points, we give the descriptions of others, so far as we are acquainted with them. Following these detailed descriptions, we then give an account and a discussion of the various views which have been held with regard to the relations of the two formations concerned, and then close with our own conclusions on this subject, as also in general upon the origin of the phenomena observed along the contact line.