Page:The Journal of geology (IA journalofgeology11893univers).pdf/166
during a trip among the dykes and sheets of the north shore of the lake, were incorporated in a monograph and published under the auspices of the U. S. Geological Survey in 1883.
The greater portion of the volume is concerned with the discussion of the Keweenawan rocks, but a brief synopsis of the character of the Huronian Series is given (pp. 367-409), and in this a few descriptions of Huronian basic eruptives are communicated. A brief synopsis of Irving's results will serve to give an idea of the relations of the different basic rocks to each other, and at the same time will serve as a basis for the present paper.
The original basic rocks of the Keweenawan, according to Irving, embrace gabbros and diabases, an anorthite rock consisting almost exclusively of anorthite, malaphyres and amygdaloids. The rocks described under the various names possess in general the characteristics of the respective types as defined by Rosenbusch in the first edition of his Massige Gesteine. The gabbros are coarse-grained rocks with a dark-gray or black color in the least coarse-grained varieties, and a light-gray color when the plagioclastic ingredient becomes greatly predominant as is apt to be the case in the coarser kinds. Their texture is highly crystalline, and their specific gravity varies between 2.8 and 3.1. The fine-grained basic rocks, whose ordinary type is diabase, make up relatively thin flows, that are almost invariably furnished with vesicular or amygdaloidal upper portions. Externally the diabases are dark in shade, being black, purple, dark green or brown, according as the rock has undergone more or less alteration. In texture they vary from medium fine-grained to cryptocrystalline. The coarser kinds grade into coarse-grained gabbros, but this gradation has never been observed in any one bed. Moreover, the diabases have undergone a great deal more alteration than the coarser gabbros, and are very strongly marked by their external characteristics, both in their fresh and altered states. They therefore seem to Irving to deserve a special name; since they possess the structure of diabases he calls them by this designation. The olivine-free diabases of the ordinary type pass into still finer grained kinds of a black or brown color. Some of these are