lawson.] Plumasite, an OUgoclase- Corundum Bock 



225 



to form edenite. The degree of serpentinization is greater than 

 would be inferred from an inspection of the thin slides. 



Original Edenite. — Turner regards the amphibole in the rock 

 described by him as secondary, but in the rock with which we are 

 here concerned it appears to be as much an original mineral as 

 the olivine. It is in part intergrown with the olivine in parallel 

 intergrowth and its idiomorphic form, contrasting with the 

 allotriomorphic character of the olivine abutting upon it, shows 

 that for the most part it antedated in crystallization the latter 

 mineral. It appears then clearly that the country rock in which 

 the eonundiferous dyke occurs is a peridotite and may for con- 

 venience be designated an amphibole-peridotite. 



In the more altered facies of this rock taken from the same 

 locality and the same mass, serpentine is the predominating 

 mineral and there arc only residuals of the olivine and pseudo- 

 morphs of the edenite. There is naturally much more secondary 

 magnetite, and a notable amount of calcite or dolomite in ragged 

 patches occurs mixed with the serpentine. 



Extent of the Oorundiferous Dyke. — The dyke of cornndiferous 

 rock which cuts this amphibole-peridotite is of quite limited 

 extent so far as the writer's observation goes. The strike of the 

 dyke is about N.N.W. or transverse to the axis of the ridge upon 

 which it is found. There are but three exposures and these do 

 not extend for more than 125 feet along the strike. The width 

 of the dyke is about 15 feet, but its dip is difficult to determine, 

 owing to the imperfection of the exposures. The slope is 

 mantled with soil and with fragments arising from the disintegra- 

 tion of the amphibole-peridotite and the exposures of the dyke 

 project through this covering of loose material. The rock of the 

 dyke is composed chiefly of feldspar and is white in color, being 

 thus in marked contrast to the darker rock mass which it cuts. 



The Oorundiferous Facies. — From a petrographical point of 

 view the rock of the dyke is far from uniform. In the middle 

 exposure, where a pit had been sunk by Mr. Edman at the time 

 of the writer's visit, it consists of a coarse allotriomorphic gran- 

 ular aggregate of white feldspar in which are imbedded crystals 

 of corundum. The feldspar is but slightly decomposed and 

 in thin section is seen to be finely striated due to twinning 



