1866.] the Western Himalaya and Afghan Mountains. 101 



ing in the map. It becomes also amorphous and forms bands of 

 considerable thickness of opaque quartzite, crossing the rocks in the 

 same manner as similar bands often cross beds of shales or other 

 stratified rocks. 



The mica is also scarce in some specimens, small spangles being 

 occasionally imbedded in the substance of the large crystals of albite 

 (c) or sparingly disseminated in the paste. But in other portions 

 of the porphyry it becomes very abundant, forming tufts of plates 

 which resist decomposition better than the other minerals, and stick 

 out of the rock where this has been worn and rounded by exposure. 

 These tufts of mica often form irregular bands. 



The augite varies from a few specks to laminar masses of 

 considerable size. It is often found associated with felspar alone, the 

 other minerals having disappeared, and it thus forms a rock composed 

 of amorphous grains of albite and lamellar masses of augite. Before 

 the blowpipe it fuses only in places, small globules of a shining black 

 glass appearing on the assay. 



The garnets are sometimes wholly wanting and sometimes very 

 abundant. It is very difficult to extract them from the mass, owing 

 to their brittleness. They are mostly found where the porphyry is 

 well crystallized and the mica abundant. 



The large crystals of albite vary in size from half an inch to five 

 inches. They have two cleavages, one nearly at a right angle to the 

 surface of the plate, or forming with it an angle of about 95°. The 

 other cuts the first cleavage obliquely with an angle of about 115°.* 

 The form of the crystals is, I fancy, uncommon, and I will describe 

 one of them with its dimensions, in order to give an idea of the 

 proportions of the crystals. , 



The crystal is always twin or composed of two hexagonal plates 

 (fig. 2) two and half inches in diameter between opposite angles, and 

 0.4 inch thick. Either four or the six edges of the plate are 

 bevelled by oblique facettes, which form with the plane of the surface 

 an angle of about 138°, so that one surface is considerably smaller 



* The angles of these crystals were measured with strips of paper and 

 a graduated half circle ; the crystals were also much weathered ; the results 

 are therefore mere approximations. If I had had the means of measuring the 

 angles with precision, I would have figured the crystals. 



13 



