Cjiapteu v. — Cryslalliiie Roch. 



Of rocks of igneous or quasi-igneous origin, two classes are represent- 

 ed within our area, namely : — 



[a.) Trap-rocks. 



[b.) Granites (quartz veins). 



Rocks of the first class are extensively developed in numerous dykes 

 which traverse the country pretty generally in various directions, while 

 those of the second class are confined more especially to the southern part 

 of the area. 



{a.) Trap-rochs. — By far the greater number of the dykes met with 



consists of coarse-grained black or blueish (or green- 

 Trappean rocks. . 



ish) black, and very hard, tough basaltic trap, but 



without olivine, except in two eases to be mentioned presently. 



Associated locally with this rock, as thin strings running into the 

 adjacent gneiss, and often even permeating the coarser rock itself, is a fine- 

 grained blueish-black trap, which is generally more split up by jointing 

 than the basaltic, or coarser, variety. 



Columnar structure has not been obsei*ved well marked, but jointing 



is often developed to a great extent, frequently pro- 

 Columnar structure, 



ducing polygonal blocks, which sometimes pass 



into the cannon-ball form so frequently seen in weathered dykes. The 



same systems of jointing as are seen in the gneissic rocks are, with slight 



variations, seen in the trap. 



Greenstone or Diorite was observed in several cases in which the rock 

 Avas markedly porphyritic. 



One case was observed at the edge of a spread of laterite about a mile 

 south-east by south of Thooanagoody " Travellers^ Bungalow,'^ on the road 

 between Trichinopoly and Tanjore. The other about 6 miles south- 

 south-west of Trichinopoly, associated with some large veins of granite 

 and quartz. In the country drained by the Vellaur and its tributary, 

 the Pereyaur, there are two principal separate series, or groups of trap 

 dykes. 



(338) 



