Lawson.1 



Orbicular Qabhro. 



887 



hypersthene, the latter two minerals being' quite abundant and 

 nearly equal in amount. The olivine is usually encased in an 

 irregular envelope of secondary, fibrous, green hornblende, 

 separating it from the feldspar. All the minerals are fresh. 

 Original green hornblende occurs in quite sparing amounts, 

 playing here the role of an accessory together with a little opaque 

 iron ore. Locally the normal rock of the mountain passes into 

 distinctly laminated facies. A sample of this fades taken for 

 study proves to lie a troctolite. It is finer grained than the 

 normal facies and presents, even in the hand specimen, an even 

 lamination, breaking under the hammer into slabs. This lamina- 

 tion is due to an alternation of layers poor in ferro-magnesian 

 constituents and chiefly feldspathic with thiner layers in which 

 the dark constituents abound. This alternation is best seen on 

 weathered surfaces. The feldspathic layers are from one quarter 

 to one eighth of an inch thick. 



In thin section the rock appears as an allotriomorphic granular 

 aggregate of basic plagioclase, entirely fresh, enclosing numerous 

 crystals of fresh olivine, more or less blurred in outline and 

 having in every case a reaction rim of secondary green hornblende 

 encasing it and separating it from the feldspar. These reaction 

 rims are sometimes of even width but are for the most part very 

 jagged, penetrating both the feldspar and the olivine and 

 evidently formed at the expense of both minerals. There are 

 besides some granules of opaque, black iron ore, usually associated 

 with the olivine, and original green hornblende occurs very 

 sparingly in small allotriomorphic grains. The laminated struc- 

 ture is not apparent in the thin sections. 



A quite different facies of the rock is one which occurs to 

 some extent as small, vaguely defined areas in the midst of the 

 normal gabbro but which is for the most part found in the 

 form of narrow veins or dykes, corresponding in character 

 to the so-called segregation veins of various plutonic rocks. In 

 these veins the rock consists almost entirely of large well formed 

 crystals of black hornblende, disposed roughly normal to the 

 walls of the vein with a little interstitial feldspar; and in the 

 narrower veins, up to about four inches in width, the crystals may 

 span the space from wall to wall. In other cases, particularly 



