64 HENRY S. WASHINGTON 



It will be seen that the rock is decidedly more basic than the other 

 diorites of the region, so far as I am acquainted with them. The silica 

 in fact is lower than that of the diabase and essexite, and closely 

 approaches that of the gabbro from Nahant. This being the case, and 

 the characters otherwise corresponding, the rock is not a diorite in the 

 proper sense, as used by Brogger, 1 but should be called a hornblende- 

 gabbro. It was thought at the time of the microscopical examination 

 that the rock was basic, but it was not expected that it would turn out 

 to be so low in silica as the analysis shows. At the same time the char- 

 acter of the hornblende, which is essentially a barkevikite, the rather 

 high alkalies, and the association with essexite and foyaite show that 

 these hornblende-gabbros ("hyperitic diorites") are decidedly distinct 

 from the other diorites, and approach more closely the essexites and the 

 more soda-rich rocks of the region. Attention has already been called 

 to the fact that they grade into the essexites, and that Rosenbusch 

 grouped them with these, but their decidedly lower alkali content and 

 lack of orthoclase and nepheline sufficiently distinguish them. 



It is to be remarked that these rocks resemble very closely under 

 the microscope some of the gabbros of Norway, especially those from 

 the district of Gran, and above all, some from the Viksfjeld. This is 

 seen on microscopical comparison of sections of the rocks, some being 

 mutually indistinguishable, and is also shown by the analyses, one of 

 those of the Gran rocks being given in II for comparison. 



H. S. Washington. 



1 W. C. Brogger, Die Eruptivgesteine des Kristianiagebietes, Vol. I, p. 93, 1894, 

 and Vol. II, p. 35, 1895. 



