COKUNDUM IN IGNEOUS ROCKS. 43 
and in the form of small veinlets or patches. In this case, however, 
the luster is more waxy and the color somewhat greenish. The 
mineral is foliated, and the scales, when examined under the micro- 
scope, show it to be uniaxial and positive and to have a feeble double 
refraction. Its specific gravity was found to be 2.74. All hough 
these data are not sufficient to identify this mineral, they resemble 
closely the characteristics of some one of the chlorites, and, from the 
analogy of the common occurrence of the chlorites with the corundum 
in the basic magnesian rocks throughout the Appalachian region 
where these rocks are abundant, these data seem to indicate that this 
mineral is one of the chlorites. 
In describing the occurrence of the corundum. Professor Lawson « 
states that the peridotite rock nuiy be found both in a fairly fresh 
condition and also largely serpen tinized. 
The fresh rock has a well-marked but nide sehistosity. On fractures trans- 
verse to this sehistosity it is of a dull, greenish-gray color and compact texture, 
relieved by long, narrow blades of a light-colored, cleavable mineral and show- 
ing irregular partings in the plane of sehistosity. On the cleavage surfaces 
the rock presents a silver-gray, spangled appearance as a groundmass, with 
numerous cleavage blades of the same light-colored material. These blades 
lie with their axes of elongation rudely parallel to the sehistosity. 
When examined in thin section, the rock is seen to be made up of 
but two primary minerals. The more abundant of these agrees with 
olivine in its optical properties and forms a mosaic of rather angular 
or occasionally subrounded anhedrons. These are traversed by irreg- 
ular sharp cracks, along whicli incipient serpentines may be observed; 
this process has occasionally proceeded so far as to give rise to distinct 
patches of serpentine. The other mineral observed lies in the mosaic 
of olivine in the form of colorless, elongated prisms, and has the 
optical characters of tremolite. This mineral makes up about 20 
per, cent of the rock. The chemical analysis of the rock ^ leaves no 
doubt of its peridotitic character and can probably be best referred 
to an amphibole-peridotite similar to those occurring in North Caro- 
lina and Georgia. Some portions of this rock have been altered to 
such an extent that serpentine is the predominating mineral. 
According to Professor Lawson's observations — ' 
The dike of corundiferous rock which cuts this amphibole-peridotite is of 
quite limited extent. The strike of the dike is about NNW., 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 dike is about 15 feet, but its dip is difficult to determine, owing to the 
imperfections of the exposures. The slope is mantled with soil and with frag- 
ments arising from the disintegration of the amphibole-peridotite, and the 
"Bull. Dept. Geol. Univ. CaUforuia, vol. 3, No. 8, pp. 222, 223. 
Mbid., p. 224. 
« Ibid., p. 225. 
