222 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



all across the belt, it is believed that the true sill form, rather than the cushion 

 form of the laccolith, is characteristic of all the intrusions which follow bedding 

 planes. The large irregular igneous mass whose western contact crosses the 

 Boundary line at a point seven miles east of the Moyie river, is, in part, a 

 cross-cutting body; its north-south arms are in sill relation, while the east-west 

 band seems to be in the form of a huge dike. 



USUAL, COMPOSITION OF THE INTRUSIVES. 



Throughout both mountain ranges the main mass of each intrusive body is 

 composed of a notably uniform type of rock. Macroseopically, the type has 

 the habit of a dark greenish-gray hornblende gabbro of medium grain. Already 

 in the hand-specimen it can be seen that hornblende and feldspar are the essen- 

 tial constituents and that the former dominates in quantity. Occasional glints 

 of light from accessory pyrrhotite may be observed. The hornblende forms 

 elongated prisms from 1 mm. to 3 mm. or more in length. They generally 

 lack the usually high lustre of the amphiboles occurring in plutonic rocks. The 

 whitish feldspars and the accessories together form a kind of cement for these 

 prisms. The principal variations in macroscopic character are due either to 

 the local coarsening of grain, as so commonly seen in gabbros, or to a likewise 

 frequent, local development of a phase richer in hornblende and poorer in 

 feldspar than the type. In the latter case the rock becomes almost peridotitic 

 in look. 



On examination of the rock in thin sections the list of constituents is 

 enlarged by the addition of titanite, ilmenite or titaniferous magnetite, pyrrho- 

 tite, apatite, rare zircons, and never-failing, though variable amounts of acces- 

 sory, interstitial quartz. Accessory biotite and orthoclase were found in many 

 specimens. 



The amphibole was found to have characters which changed rather regu- 

 larly with the freshness of the rock. In the freshest specimens it was a compact, 

 strongly pleochroic mineral with the following scheme of absorption : — 



Parallel to a — light yellowish-green. 

 " "b — strong olive green. 



" c — deep bluish-green. 



b>c>a 



In specimens which appear to have been slightly altered, the hornblende 

 is still compact but the colours are considerably paler, so as to give the mineral 

 the look of actinolite. A further stage of alteration ig represented in a fibrous 

 phase of the amphibole, suggesting uralite in colour and other essential respects. 

 This fibrous amphibole is so common in the slides that it was at first believed 

 that it might be secondary after a pyroxene. A close study of a large number 

 of thin sections has, however, led to the conclusion that the fibrous amphibole 

 is really secondary after the compact form. All stages of transition can' be 

 found between the two, and the fibrous type has demonstrably grown at the 



