lawson.] Plumasite, an OUgoclase- Corundum Bock. 



223 



tion as regards the tremolite by saying: "The colorless 

 amphibole was at first supposed to be tremolite, which is a 

 lime-magnesia amphibole. Chemical analyses and microscopic 

 examinations, however, show that there are two colorless 

 amphiboles present, one monoclinic probably edenite, and the 

 other orthorhombic probably gedrite." The samples of rocks 

 collected by the present writer at the place where the corundifer- 

 ons dyke cuts it agree fairly well with Turner's description 

 except as to minor details. Here the rock maybe found both in 

 a fairly fresh condition and also largely serpentized. The fresh 

 rock has a well marked but rude schistosity. On fractures 

 transverse to this schistosity it is of a dull greenish gray color 

 and compact texture, relieved by long narrow blades of a light 

 colored, cleavable mineral, and showing irregular partings in 

 the plane of schistosity. On the cleavage surfaces the rock 

 presents a silvery gray spangled appearance as a ground-mass, 

 with numerous cleavage blades of the same light colored mate- 

 rial. These blades lie with their axes of elongation rudely 

 parallel to the schistosity but in all orientations in that plane. 

 They are about 5X1 mm . in size . 



Examined in thin section the rock is seen to be made up of 

 but two primary minerals. The more abundant of the two 

 agrees with olivine in its optical properties. It forms a mosaic 

 of rather angular or occasionally subrounded anhedrons, which 

 have an average size of .25 mm. Occasionally these show a 

 tendency to elongation, so that one diameter may be twice the 

 other, but for the most part they are approximately equidimen- 

 sional in cross section. The olivine is not only a mosaic 

 aggregate of anhedrons in this section, but many of these anhe- 

 drons have a polysomatic structure. The anhedrons, whether 

 simple or polysomatic, are traversed with irregular sharp cracks, 

 and along these cracks incipient serpentinization may be observed 

 as noted by Turner. Occasionally this process has proceeded so 

 far as to give rise to distinct patches of serpentine. Lying in 

 this mosaic are elongated prisms of a colorless mineral having 

 the optical ehai'acters of tremolite. This makes up about 20 per 

 cent, of the rock. In the zone of the vertical axis it has idio- 

 morphic boundaries, but the ends of the prisms are splintered 



