THE TERRESTRIAL PERIDOTITES. — LHEEZOLITE. 135 



occur similar to those observed in the olivine of the Siberian paUasite {ante, p. 72). 

 These tubes lie parallel to the plane of extinction in the olivine. 



The grayish-white groundmass is composed of eustatite, a little diallage, and various 

 secondary products. The enstatite varies from a clear transparent mineral to a pale- 

 brown and a reddish-brown. The last is so associated with the first as to indicate that it 

 is a partially altered state of the first. This reddish-brown portion owes its color to the 

 same included plates as those commonly seen in bronzite and hypersthene ; and it could 

 be well called the former, or, because the mineral is feebly dichroic, the latter. Whether 

 the altered portions of the groundmass are derived in part from the alteration of feldspar 

 or entirely from the pyroxene minerals, is not known. No distinguishable feldspar was 

 seen. The pyroxene minerals are in part changed to serpentine, and in part to an indefi- 

 nite aggregately polarizing mass, usually surrounded by one or two bands of secondary 

 minerals standing perpendicular to the bounding surface. The outer band possesses a 

 polarization similar to that of enstatite, while the inner resembles an amphibole mineral. 

 In some cases on the olivine side, another band composed of serpentine was observed. A 

 similar structure frequently exists in altered gabbros, but in this case the products are 

 not sufficiently well defined to be determined. Considerable dolomite was seen in the 

 more highly altered portions of the rock. Altered picotite or chromite (slightly translu- 

 cent and reddish-brown in spots), as well as iron ore produced during the conversion 

 of the olivine into serpentine, was found. Only a small portion of this ore remains in 

 the parts where the conversion of the entire olivine mass is most comiilete. 



Figure 3, Plate VIII., shows the general structure of this rock. The white portions 

 have the granules still unchanged in the greenish secondar}^ serpentine material, which in 

 part replaces the original olivine. Tlie brownish portions forming a matrix for the altered 

 olivine are the enstatite and diallage, which for the most part are changed. The bluish 

 band on the right is one of the alteration-borders between the olivine and enstatite. 



Gjtprud, Norway. 



This, according to Mohl, is principally composed of rounded and obtuse-angled olivine 

 grains, which form from sixty per cent to seventy per cent of the mass. 



Along the contours and fissures the olivine is changed to a bright leek-green, gray-green, 

 and grass-gi-een chrysotile, whose fine fibres are arranged partly across, and partly 

 parallel to, the direction of the veins. The centre of tliese chrysotile veins is generally 

 filled with a fine black line of magnetite or by aggregations of magnetite grains. 



Some hypersthene or an augitic mineral occurs in the section. This is dichroic, of a 

 chocolate-brown color, but altered in ])art to a cloudy-grayish and yellowish-white sub- 

 stance, supposed to be a magnesium carbonate.* 



In a section of Gj(/)rud serpentine purchased from E. Fuess, a gray, sponge-like mass is 

 seen holding the greenish, serpentinized olivine. Excepting that the alteration has pro- 

 gressed some further, the description of the Christiania peridotite would apply to this 

 section. If the dichroism of the rhombic pyroxene arises from alteration, as I suspect it 

 does, there seems to be no reason for calling it hypersthene, instead of enstatite. 



Its structure and alteration are .shown in figure 4, Plate VIII. 



* Njt Mag., 1S77, xxiii. 122, 123. 



