﻿ASBESTOS AND MANGANESE DEPOSITS. 151 



unaltered, but it occurs in all stages of alteration, from the one showing 

 the mere beginning of chemical change, to a rock wholly serpentine. I 

 first encountered this rock on the summit of the plateau at an altitude of 

 2,000 feet, northeast of Pasuquin, where it is fairly fresh. The forma- 

 tion is again found near Bangui, and on Monte Inmenso it is little altered, 

 but in many places in the intervening country it is highly serpentinized. 



Description. — In the field the rock is black, usually crumbly, and is 

 characterized by innumerable, small veins of magnesite and surface 

 incrustations of the same; it imparts a very dreary aspect to the country 

 where it is the surface rock. 



The hand specimen is quite dull, compact and fine grained, however 

 with brighter patches of from 2 to 5 millimeters in diameter, rarely more. 

 These are the rhombic pyroxenes, bronzite and hypersthene, more com- 

 monly the former. The luster is due to "schillarization," a well-known 

 characteristic of these minerals in the basic rocks of this type. 



Microscopic. — In thin section in ordinary light there are large areas 

 of a yellowish, glassy mineral to be seen, full of cracks and veinlets, 

 forming a mesh structure; occasional irregular sections of a colorless 

 but dirty mineral, marked by very fine and close prismatic cleavage are 

 observed, and finally many irregular patches of magnetite and hematite 

 occur. The first mineral is enstatite, characterized by its low double 

 refraction ; the fibrous material in the cracks forming the meshes is 

 serpentine; the large sections with close cleavage and parallel extinction 

 constitute bronzite, in part changed to bastite. 



This slide exhibits one of the four characteristic alterations to ser- 

 pentine. Below is an analysis 3 of this rock : 



Analysis. 



Per cent. 



Si0 2 37.58 



ALO : , .57 



Fe,0 ; , 9.49 



MgO 32.34 



CaO .48 



Na,,0 .32 



KX> .20 



H 2 O+110° 6.70+100 



H,0 — 100° 12.64+110 



TiO, Trace. 



PA None. 



MnO .28 



THE GRANULITE (MUSCOVITE GRANITE). , 



Up to this time this rock has been encountered only in three localities 

 in this province — at Dalumat, the southernmost point, in the headwaters 

 of Caraon River, and at Baruyen Hill, close to the north coast of Luzon. 



3 Analysis by L. A. Salinger, Chemical Division, Bureau of Science. 



