32 FOOTE : GEOLOGY OF MADURA AND TINNEVELLY DISTRICTS. 



is the almost entire absence of intrusions of trappeau rocks which aie 

 ^ Absence of trappean SO common in more northern parts. Only three 

 intrusions. trappean instrusions came under my notice in the 



south, of which only two were of trap rocks in situ. These were a tiny 

 djke of diorite a few inches thick and a few 



Only 3 cases seen. 



yards long running nearly due north and south 

 exposed in the dry bed of the Tumalpadi tank south of Tirushulai in 

 Ramnad zemindari. The second case is in the narrow coast strip of 

 g-neiss at the south end of the Cape Comorin base line. Here a number 

 of large weathered bloclcs of diorite are scattered about among the 

 blown sand hillocks. They looked as if they had been much surf-worn 

 and were probably derived from the marine beds, the remains of which 

 stretch away to the northward. The third ease did not occur actually 

 within the limits of the present Memoir, but in the Travancore country a 

 few miles to the westward. Here a very narrow sharp cut dyke of tachy- 

 lite is seen cutting through massive granite gneiss. The dyke which is ex- 

 posed for a distance of between 100 and 150 feet in length is only 4 or 5 

 inches thick. It has weathered somewhat faster than the granite gneiss it 

 cuts through, and is therefors rather sunk, and forms a small channel 

 across the face of the rock. 



Granite and quartz veins are also of rare occurrence throughout the 



Rarity of granite and southern gneiss area east of the Ghats, and mostly 

 quartz veins, £^j. ^^^ small in size to admit of their being shown 



on the map, or to be worth enumerating in this memoir. The largest in 

 point of size is one already adverted to (page 16) as occurring on the 

 summit of the Perumal Malai ridge 13 miles north-east of Madura, where 

 it has been irrupted in the axis of a very sharp anticlinal fold. The granite 

 is a pale flesh-coloured binary compound of quartz and orthoclase felspar. 

 Some fair-sized veins of a ternary granite may be seen catting 



Granite veins in Triva- across the granite gneiss ridge close south of the 

 dur ridge. Trivadur Trigonometrical Station 5^ miles south- 



south-east of the Perumal Malai. These veins occupy planes of jointing 

 with a southerly dip crossing the strike of beddings nearly at right angles. 

 ( 32 ) 



