CHOPAKA BASIC INTRUSIVES 341 



Most of the rock within the area is feldspathic and seems to belong to 

 a fairly steady type — normal gabbro transitional to metagabbro. It is 

 a dark gray-green, medium grained, hyp idiom orphic granular rock, orig- 

 inally composed of essential labradorite (At^Ai^) and diallage and acces- 

 sor}^ apatite, with a little magnetite. Crush metamorphism, supplemented 

 by ordinary weathering, has largely changed the diallage into actinolitic 

 amphibole, both compact and smaragditic. The specific gravity of the 

 least altered rock is 2.959, taken, as were all the other specific gravities to 

 be mentioned, at room temperatures. 



That common rock type is associated with a large body of a dark green- 

 ish gray, fine grained rock of which several specimens show the composi- 

 tion very clearly. It was originally made up entirely of granular olivine 

 without any certain accessory constituent. No trace of chromite has been 

 found. Serpentine, talc, tremolite, and magnetite are present in most of 

 the thin-sections, but apparently in all cases as decomposition products of 

 the olivine. The specific gravity of the rock varies from 3.100 to 3.173. 

 It is a dunite without chromite. 



The field relation of the gabbro and olivine rock has not been deter- 

 mined. They may belong to distinct intrusions or they may be due to 

 differentiation within a single body. Though transitions seemed to ap- 

 pear in the actual outcrops, the search for final evidence in these rocks, 

 crushed and obscured as they are, has so far proved unavailing. Analo- 

 gous occurrences in other parts of the boundary belt suggest that gabbro 

 and olivine rock were intruded at different dates. 



ASHNOLA GABBRO 



Throughout its 5 miles of length the Ashnola gabbro body is homo- 

 geneous in composition, but often varies abruptly in grain from medium 

 to quite coarse. The color is uniformly a peculiar deep fawn, which is 

 the dominating tint of the feldspar. This color is rather remarkable, as 

 the rock proves under the microscope to be quite fresh, with feldspars of 

 glassy clearness. The essential constituents are a green augite, often 

 colorless in thin-section, brownish green hornblende, brown biotite, and 

 labradorite, Ab 5 An 6 . Abundant apatite, some magnetite, and a very little 

 interstitial quartz are the accessories. The structure in the original rock 

 is the hypidiomorphic granular, though the augite is often, especially in 

 the coarser grained phases, poikilitic. Eegular intergrowths of the augite 

 and hornblende are common. The rock is an augite-hornblende-biotite 

 gabbro, the specific gravity of which averages 2.946. 



Although the gabbro is older than the Eemmel granodiorite and has 

 shared in the great dynamic metamorphism which, as we shall see, has 



