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time principally to the discovery of the relations existing between the several rocks, and made no efforts to divide these into their varieties.

With the establishment of the surveys of Minnesota, Michigan, and Wisconsin, however, an attempt was made to classify with scientific accuracy the basic rocks of these three states. Kloos^[1] had already discovered the gabbro of Duluth and had identified a melaphyre from the same place, but had made no very exact determination of either. Among the geologists on the Michigan and Wisconsin surveys, Messrs. Julien, Wright, Wichman, Pumpelly and Irving examined microscopically the rocks of the Huronian and the Keweenawan series of Wisconsin, and of the Archæan, Huronian and Keweenawan of Michigan, and among the descriptions of these rocks which they give may be found very exact accounts of the characteristics of the diabases, gabbros and other basic eruptives of the region.

Messrs. A. A. Julien^[2] and C. E. Wright,^[3] as early as 1873, mentioned quite fully the greenstones and traps of the Archæan and of the iron-bearing formations in Michigan. The former writer identified many massive and schistose rocks to which he gave the name diorite, since he found in them hornblende, but no augite. Mr. Wright likewise discovered hornblende rocks which he evidently regarded as original, since he calls them all diorites. Mr. Wright's determinations are the first ones based upon microscopical observations of Lake Superior rocks. Messrs. Brooks^[4] and Pumpelly^[5] contented themselves with macroscopic examinations of the basic rocks of the iron and copper-bearing series in this state, and in this way distinguished diorites, melaphyres and amygdaloids, while Mr. Marvine^[6] divided the