INTRUSIVE ROCKS IN THE GNEISSIC AREA. 43 



Musi river ; the remaining few are scattered about here and there at 

 very great distances from each other. The dykes are all dioritic in 

 character, and many of them, especially in the northern group, distinctly 

 porphyritic, showing numerous whitish felspar crystals. The diorite 

 is mostly blueish or greenish black in colour. Exceptions to this rule are 

 the dykes occurring at Ongole (on the Trigonometrical Station hill) , 

 and Neputlapadu, 3 miles east of the southern end of Chimakurti 

 mountain. The former is of greenish-whitish grey colour, the latter of 

 a purplish-blackish grey. 



The majority of the dykes may be referred to two systems, depend- 

 Keferred to two sys- in S u P on the direction of their strike, one of 

 tems - these systems having a course from north- 15°- west 



to south-15°-east, the other running north-east-by-east to south-west- 

 by-west. 



All these dykes were intruded prior to the deposition of the Kadapa 

 rocks. 



b. — Granite veins. »' 



Granite veins, except of very small size, are not at all common in the 

 gneissic area, and none were met with of any importance either from 

 size or special geological interest. 



Some small veins of quartzo-micaceous granite traversing the 

 schistose gneissics to the south-west of Kambaldinna, in the central part 

 of the valley of the Man-eru (river) , contain small garnets and prisms of 

 tourmaline. Tourmaline occurs also in granite veins south-east of 

 Petlur (18 miles south- west-by -west of Ongole). The tourmaline occurs 

 here in some quantity, and good prisms are obtainable. Hemihedral 

 crystals are not common, and the colour of the tourmaline is always 

 black. A granite vein containing unusually large crystals of orthoclase 

 felspar was observed in the bottom of the great tank west of Kondapy, 

 at its upper end. Well shaped prisms of very pale flesh colour, 4 to 6 

 or 8 inches long and proportionately thick, were noticed, but were 

 mostly too much cleaved to be extricable without breaking up into small 

 fragments. 



( 43 ) 



