8l6 THE JOURNAL OF GEOLOGY. 



suggest the obscure relationships of the rocks designated by 

 them. 



A rapid glance at the sections of the rocks composing the 

 beds indicates that they may be separated into three classes, of 

 which one, the non-feldspathic gabbros, corresponds to the 

 peridotites 1 associated with the Scottish gabbros. The other two 

 classes embrace granulitic rocks whose structure is similar in all 

 essential respects to the structure of the Scottish rocks referred 

 to above. 



The members of one of these two classes are granulitic 

 gabbros, corresponding in their important features with the 

 granulitic gabbros of Professor Judd. The other class of granu- 

 litic rocks comprehends a series of quartzose members, composed 

 of quartz and olivine, of quartz and hypersthene, or of the three 

 minerals mentioned. The quartz is evidently not secondary, for 

 the olivine associated with it is almost entirely unaltered in many 

 cases. It has crystallized in its present position, and in most 

 specimens after the olivine was formed. It has been suggested 

 that these peculiar rocks were originally quartzites that have 

 been altered by the great gabbro mass south of them. Many of 

 them appear to have had this origin. They are probably meta- 

 morphosed sediments, but of such a unique character that a 

 critical study of them is demanded before a positive decision as 

 to their genesis can be given. 



The present communication deals only with the basic and the 

 granulitic gabbros, rocks that are unquestionably phases of the 

 normal gabbro. 



B. The Non-Feldspathic Gabbros ^Peridotites). 



In choosing the group name " non-feldspathic gabbros " for 

 these rocks, in preference to the more usual one "peridotites," 

 emphasis is placed on the fact that the rocks included under it 

 are nothing more nor less than gabbros in which feldspar is 

 lacking. The magma which gave rise to them was undoubtedly 

 a portion of that which, under the ordinary circumstances 



'J. W. Judd: Quart. Jour. Geol. Soc .-Vol. 41, 1885, p. 357. 



