1836.] 



hetween Madras and the Neilgherries. 



7 



After passing the eastern Ghauts, the rocks met with on the road to 

 Baitmungalum are gneiss, the contorted strata of which are seen 

 almost in every one of the blocks (No. 27). 



This gneiss is similar to the same rock found in many other places 

 in India, and, like it, has some of its strata almost exclusively formed 

 of mica, with a few grains of quartz ; and, in this last case, a hand 

 specimen of it might be taken for mica slate (No. 28). 



This last mineral (like hornblende in the sienitic granite) is occa- 

 sionally contained in nests in this gneiss; and, mica being easily de- 

 composed, it falls off, leaving small cavities on the surface of the rock, 

 which are seen in many of the masses about Baitmungalum. 



Going a mile or two west, and before coming to Golcondapatnam, 

 the granitic blocks are scattered about all over the plain, particularly 

 near the last mentioned village; they look like erratic blocks, and are 

 easily recognized as granite by their appearance, different from those 

 of gneiss, which have hardly any elevation above the soil, with a 

 rounded convex surface ; while the granitic clusters are more elevated 

 and affect either a prismatic form, or are piled up one on the other like 

 logging stones. Those close to Golcondapatnam, at the foot of which 

 some batteries have been erected, have this last appearance. 



In the dry bed of a river, before arriving at the last mentioned vil- 

 lage, I saw a thick basaltic dyke, which stretched across the whole 

 breadth of the river, its outgoings being split into rhombs, or paral- 

 lelopipeds. This dyke appeared to have burst through the granite 

 (No. 29). 



Looking attentively to the numerous blocks of granite scattered 

 about this plain, particularly those about a mile or two north of Gol- 

 condapatnam, the idea presents itself, almost involuntarily, to the mind, 

 whether these masses are not erratic boulders, similar to those found 

 in the plains of Sweden, Russia and northern Germany, and which, it is 

 now ascertained, were derived from the Scandinavian mountains, very 

 likely at the time of their elevation. 



Brongniart (a) is of opinion that the boulders near Hyderabad are of 

 the erratic kind, quoting the authority of DeLuc (nephew) ; and, really, 

 seeing on a level and perfectly horizontal plain (no hills of any magni- 

 tude being within many miles of them) these sometimes solitary, 

 unconnected boulders, scattered in the most irregular manner, and, at 

 others, heaped together confusedly in clusters and groups, the opi- 

 nion of some geologists, regarding the erratic nature of these boulders 

 in India, does not seem improbable. 



This granite exfoliates in concentric laminae of different thickness ; 

 nor are the causes producing this exfoliation confined to the mere sur- 



(a) Tableau des Terrains, &c. page 83, 



