J30 VREDENBURG: SKETCH OF BALUCHISTAN DESERT. 



cast-west. The intrusive character of the rock is made clear by the 

 apophyses which it sends into the surrounding rocks and the inclusions 

 of sedimentary rocks all round the margin of the igneous mass. The 

 sedimentary rocks are altered by contact metamorphism, and the 

 intrusive rock itself is of a different character near the boundary, 

 becoming finer grained and less perfectly crystalline. 



The minerals present in this rock are orthoclase and oligoclase 

 felspar, augite, hornblende, biotite, magnetite (or ilmenite), sphene and 

 apatite. The two latter minerals are very abundant and conspicuous 

 even to the unaided eye, the apatite often forming beautiful 

 crystals of a pale sea-green colour, which frequently reach a dimen- 

 sion 5 millimetres in diameter. Throughout the enormous area occu- 

 pied by the plutonic mass the same minerals are found, but their 

 proportion varies greatly from place to place. In the eastern portion 

 of the intrusion the rock contains a considerable proportion of large 

 crystals of orthoclase. Augite is much more plentiful than 

 hornblende, so that the rock should be classed as an augite syenite. 

 Further west the proportion of hornblende 



Augite syenite. . . 



increases, while orthoclase is replaced almost 

 entirely by plagioclase felspars, the rock becoming a diorite. 



South and south-west of Alam KMn, at the western extremity 

 of the great intrusion where it breaks up into a number of separate 

 dykes, the structure as seen under the microscope is quite por- 

 phyritic ; and this is the case also with all the specimens collected 

 near the junction of the igneous mass with the sedimentary strata, 

 even where the outcrop is broadest. 



In these portions where the outcrop is very broad the sedimen- 

 tary rocks are altered by contact metamorphism up to a distance of 

 as much as fifty feet from the boundary of the intrusion. On its 

 northern side the intrusive mass comes into contact with ancient 

 volcanic rocks of the flysch period ; being igneous rocks themselves 

 they do not show very conspicuously the effects of contact metamor- 

 phism. But to the south where the intrusion has found its way 

 through the eocene argillaceous rocks the effects of metamorphism are 

 ( 5* ) 



