PREVIOUS DESCRIPTIONS. 123 



(p. 257). Captain Ouchterlony in 1848 referred to the mass or nuc- 

 leus of the mountains as <l granite frequently passing into sienite v 

 {op. ct't., p. 2). He draws a distinction between the u granite ", 

 "sienite" and " hornblende rock" of the Nilgiris on the one hand, 

 and the "beds of gneiss " met with in the plains on the other. 



With the systematic mapping which was commenced by the 

 officers of the Geological Survey of India in 1857, a large mass of in- 

 formation was accumulated concerning the distribution of the char- 

 nockite series, as well as other formations in South India. The 

 macroscopic and field characters of the rocks have been clearly 

 described in the reports of the work done by H. F. Blanford, C. JE, 

 Oldham, W. King, R. B. Foote and P. Lake, most of which have 

 been published in the Memoirs and Records of the department. 1 



As at the time of the issue of these publications provision had 

 not been made in the department for microscopic examination of 

 the rocks, the wide prevalence of the pyroxenes, and especially of the 

 rhombic forms, was not noticed, the dark mineral in the rocks being 

 generally referred to hornblende, which is also an abundant constit- 

 uent. Following also the theories then prevalent as to the nature of 

 the Archaean crystalline rocks, most of the authors who described th e 

 geological features of Madras regarded the gneissose and the banded 

 structures as evidence in favour of considering the crystalline rocks 

 to be the results of the metamorx)hism of sediments. But we now 

 know that neither the gneissose structure, nor the banding due to 

 differences in mineral composition of the "beds," differs essentially 

 from the phenomena presented by some rocks whose eruptive origin 

 is established beyond dispute. In the light of this modification of 

 old views concerning the gneisses it becomes necessary to separately 

 re-examine each group and record the evidences bearing on its 

 origin. 



1 See Manual, Geol. of India, 2nd Ed., pp. 36—39. 



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