2 N. L. Boiven — Genetic Features of 



a rather inconspicuous knoll upwards of 200 yards in 

 diameter lying immediately adjacent to and south of the 

 railway track about half a mile west of the station. The 

 rocks are well exposed in the knoll itself, but their 

 relations to surrounding rocks are not visible. The simi- 

 larity to the alnoites of the Paleozoic intrusives makes it 

 impossible to doubt that it belongs, with them, to the 

 alkalic rocks of the petrographic province to which Dr. 

 Adams has given the name, Monteregian Hills. 4 



Bock Types. 



The principal rock of the outcrop is dark gray, mostly 

 fine-grained but mottled by large poikilitic biotites about 

 1 cm. in diameter which are the only crystals determinable 

 in the hand specimen. This type constitutes practically 

 the whole mass but there are streaks and indefinite 

 patches of a coarser-grained type in which biotite and a 

 light-colored constituent are readily distinguishable, and 

 also, in one place, a fine-grained minette-like type as an 

 irregular dike. 



Mineral Constituents. 



Under the microscope the principal type, constituting 

 the main mass, is found to consist of biotite, olivine, 

 augite, melilite, perovskite, an opaque ore mineral, 

 apatite, marialite, and alteration products of the above 

 minerals, principally carbonates. The rock therefore 

 has the typical composition of an alnoite, that is, a mica 

 peridotite with melilite. In one important particular 

 this rock is different from all other alnoites as described. 

 It contains two olivines, the ordinary olivine, chrysolite? 

 and monticellite, the lime-magnesia olivine, a mineral 

 hitherto unrecognized except as a product of contact 

 metamorphism. 



Biotite is decidedly the most abundant constituent of 

 the rock. It occurs in large grains up to 1 cm. in diame- 

 ter that are full of corroded inclusions of chrysolite and 

 augite, visible even in the hand specimen. The biotite 



4 F. D. Adams, Jour. Geol., 11, 239, 1903. 



5 Throughout this paper it will be necessary to refer to the ordinary, mag- 

 nesia-iron olivine specifically as chrysolite in order to avoid the confusion 

 that would be involved in the use of the group name, olivine. 



