LADAKH, NORTH-WESTERN HIMALAYA. 309 



denudation, while elsewhere the process of deposition was going on 

 continuously, 



In the central portion of the exposure the sedimentary beds are 

 in direct contact with the older rocks along their south-western 

 margin, but at either extremity they are separated by a great series 

 of volcanic rocks of a very basic type. There can be no doubt that 

 these rocks, which form the upper limit of the tertiary system of this 

 region, are in the main contemporaneous eruptive products, as they 

 include beds of volcanic ash and agglomerate, 1 but there are also 

 numerous intrusive masses associated with the bedded traps. Basic 

 trappean intrusions are also found in the pre-tertiary rocks south- 

 west of the boundary, which are evidently connected with these 

 same eruptive rocks. These intrusions are interesting as, at Puga 

 and in the Markha valley south of Leh, they are composed of peri- 

 dotite, until lately the only recorded instances of ultra-basic rocks 

 having been found in India/' 2 



The microscopical examination of the specimens enumerated 

 above shows that many of them are ultra-basic peridotites, and 

 others are serpentines formed by the more or less complete alteration 

 of olivine rocks. Both groups belong to the plutonic class of igneous 

 rocks ; their structure is completely holo-crystalline, and they have 

 never flowed out on the surface of the earth as lavas. The same 

 remark applies to the gabbros. 



The association of these holo-crystalline plutonic rocks with 

 volcanic beds of lava and ash, as in the Puga valley, must be purely 

 accidental. The holo-crystalline igneous rocks are evidently, as 

 suggested by Mr. Oldham, intrusive in the volcanic series, and may 

 possibly have no direct genetic relation to them. It is quite a 

 common feature in Himalayan geology to find diverse igneous 

 rocks following each other along the same planes of weakness. 



As will be seen from my detailed description of the porphyritic 

 diorite collected by Mr. Oldham (which was not found in situ), 



1 C. A. McMahon, Records, XIX, p. 118 (1885); R. D. Oldham, Records, XXI, 

 p. 154 (1888). 



2C. A. McMahon, Records, XIX, p. 115 (1885). 



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