The Neilgherry Mountains. 



[No. 34, 



Geoio°'icai struc- geological formation of the Neilgherries is of 



{he primitive igneous order ; the mass or nucleus of 

 the mountains being granite, frequently passing into sienite. 



In every part of the Hills innumerable dykes or channels of basal- 

 tic rock, hornblende, quartz, and other minerals commonly found in 

 a similar relation to rocks of the primitive class, intersect the granite, 

 in some instances of considerable magnitude, but more commonly 

 broken into small ramified branches or veins. 



In the Koondahs trap rock is more extensively developed, being 

 often found in that part of the district capping the Hills, and spread- 

 ing out to a considerable extent ; but a description of the geology of 

 the Koondahs does not enter into this memoir. 



Hornblende rock is also found to occur in some abundance in va- 

 rious parts of the Hills, passing occasionally into hornblende slate, 

 and porphyritic hornblende, having garnets imbedded. This rock 

 is highly ferruginous and decomposes into a stiff red clay which 

 forms extensive beds underlying the soil in many parts of the Hills. 

 It intersects the granite and sienite in deep channels or dykes, which 

 having a greater tendency to decomposition than those rocks, fre- 

 quently occasion chasms, in which water lodging and wearing away 

 the sides become the cause of the disruption of large masses, which 

 are continually parting from the parent rock, especially after heavy 

 rains. 



No stratified rocks make their appearance in any part of this dis- 

 trict except at the N. E. angle of the plateau, where, on descend- 



Gneiss towards the plains, beds of gneiss are met with, 



but so torn and distorted as to render it almost im- 

 possible to derive from their occurrence any geological data of 

 value. The run of the beds however may be pronounced about 

 north and south, the dip being to the east at an angle varying be- 

 tween 30° and 60®. Near the junction with the granite which forms 

 the country in the neighbourhood, the gneiss is much altered, and 

 veins of igneous rock perforate it in all directions. 



I examined this part of the district with much interest, both on ac- 

 count of the highly metalliferous character which this rock usually 

 possesses in other countries, especially as in Saxony its occurrence is 

 marked by the same accompaniments as I have described, and in the 

 Limestone does ^^^"'^ hope that beds of primary limestone might be 



notoccui. found to occur in the vicinity ; this mineral, which 



