Page:The Journal of geology (IA journalofgeology11893univers).pdf/165

from Ashland county, Wisconsin; and Wichman^[1] published a classification of Huronian rocks based on their microscopical examination. Wichman divided the massive basic rocks into diabases, coarse-diabases and diorites. The only other microscopical work done in connection with the Wisconsin Survey is that by the late C. E. Wright, published in the second volume of the reports. In this Mr. Wright^[2] mentioned the occurrence of a diorite containing augite in the bed of Black river.

Further, Dr. Wadsworth,^[3] in his discussion as to the origin of the jasper and iron-ores of the Marquette region describes briefly the microscopic features of many of the intrusive knobs that are so prominent a feature in the topography of the district. These are declared to consist largely of diabase and coarse basalt, both massive and slightly schistose.

The investigation of the basic rocks of the region had by this time been sufficiently exact, and the number of specimens examined was large enough to give an idea of the characters of the commonest types occurring there, but these investigations had been undertaken by so many different geologists that no exact correlation between the various varieties discovered was possible. No classification of these could be accomplished until some had examined the specimens from all the different localities and had compared them with one another. This work was undertaken by Professor Irving^[4] in 1881, and was ably accomplished by him in the course of two years. All publications referring to the lithology of the Keweenawan and Huronian formations on both sides of the lake were carefully reviewed, most of the specimens described in them were examined, and the results of this study and examination, together with a great deal of new information gathered