592 THE JOURNAL OF GEOLOGY. 



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 dis- 

 cussion 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 consist- 

 ing 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 Rosen- 

 busch in the first edition of his Massige Gesteine. The gabbros 

 are coarse-grained rocks with a dark-gray or black coloi 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 rela- 

 tively thin flows, that are almost invariably furnished with vesic- 

 ular 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 grada- 

 tion 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 there- 

 fore 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 



