Chap, V.] crystalline hocks. 113 



in the greenstone dykes of Ootatoor/>^ all of which (lisapi>ear under the 



cretaceous rocks without in the least affecting tliem. Had the dykes 



been formed by intrusion of the trappean rocks after deposition of the 



beds of the Cretaceous system, the latter must inevitably have shown 



signs of alteration at the point of junction, even if they had not been 



Majority pre-cretace- penetrated themselves by the eruptive masses. 



Fi'om the great similarity of mineral character, it 



does not seem rash to argue the probability of the dykes, generally, being 



of pre-cretaceous age. 



The economic capabilities of the trap rocks will be touched upon 



in the Chapter siDCcially devoted to Economic Geology. 



The lengths of several of these important examples of intrusive 



„ , rocks have already been given, their thickness is 



Size of dykes. 



very variable, but they are very frequently so 

 situated, that either by soil or jungle great part of their thickness is con- 

 cealed, while in other cases the fallen blocks lie so thick, as to render 

 it impossible to either measure or estimate exactly the real width. This 

 is especially the case where the dykes form elevated ridges. The greatest 

 width actually measured (by pacing) at the northern end of the Pulioor 

 dyke was about 200 yards, but the average width of the dyke is only 

 about 40 yards. 



A dyke exceeding 50 feet in width would be reckoned a good sized 

 one, and one more than 100 feet across, as of first class size. Many 

 examples of small branches a yard or two, but often also much less, in 

 width were noticed, whose course was qu.ite sharply defined like the 

 foundation of a wall, and might be traced for some little distance. 



In some few cases the branches could be traced till they tapered 

 away to 1 or 3 inches in width and then died out altogether. 



(3.) Granites and (Quartz veins. — The greatest development of gra- 

 nite occurs in the Trichinopoly district, along the north bank of the 



* Memoirs of the Geological Survey of India, Vol, IV, part 1, page 37, 



p ( 335 ) 



