THE BASIC MASSIVE ROCKS, ETC. 451 



twinning lamellae. On the other hand, the pinacoidal parting 

 is entirely absent in cases where twinning lamellae are present. 

 Consequently not much dependence, can be placed upon this 

 constituent as a means of distinguishing between gabbros and 

 diabases. The former rocks are evidently related to the latter, 

 whose typically granular, holocrystalline forms they are. Irving, 1 

 in his work on the geology of the Keweenawan series in Michi- 

 gan, Wisconsin, and Minnesota, was compelled to make use of 

 coarseness of grain as a means of distinguishing between diabases 

 and gabbros, both of which were thought by him to occur as 

 flows. "It is evident," he writes, "that my observations on 

 these north Wisconsin gabbros bear out the conclusions reached 

 by certain European lithologists, as to the subordinate import- 

 ance of the foliated condition of augite, by which gabbro is 

 ordinarily separated from diabase, of which it would seem to be 

 merely a phase. Nevertheless, the name is here retained, not 

 only because most of our rock is very close to the typical Euro- 

 pean gabbros, but more especially because it is so sharply con- 

 trasted with the typical Keweenawan diabase that a separate 

 name seems necessary." And again, when speaking of the dia- 

 bases, he says, 2 "Although grading through coarser kinds into 

 the coarse olivine-gabbros. the fine-grained rocks here considered 

 deserve a place by themselves. The gradation into the coarser 

 kinds has never been observed in any one bed, and they are very 

 strongly marked by their external characteristics, both in the 

 fresh and altered states." 



The prime distinction between the two classes of rocks is, 

 then, one based upon structure and not upon the difference 

 between the augitic and diallagic nature of its pyroxenic con- 

 stituent. The structure of the most typical gabbros was recog- 

 nized by most geologists to be granitic and that of the diabases 

 as ophitic. Professor Judd 3 proposed to restrict the name 



'Geology of Wisconsin, III, 1880, p. 171. 



2 Copper-Bearing Rocks of Lake Superior, p. 69. 



3 J. W. Judd: On the Tertiary and older Peridotites of Scotland. Quart. Jour. 

 Geol. Soc, Vol. XLI, 1885, p. 354; and On the Gabbros, Dolerites and Basalts of 

 Tertiary age in Scotland and Ireland. lb. XLII, 1886, p. 49. 



