10 H. D. Oldham — Essays in Theoretical Geology. 



the rock area, and, to crown all, there are outliers innumerable of 

 the peninsular rocks standing up from the alluvial plain. 



These last contrasts between the two areas are shown even on 

 a good topographical map ; but that now to be mentioned, the most 

 important of all, can only be seen on one that is geologically 

 coloured. 



Along its southern boundary the alluvium is in contact with rocks 

 of various ages ; but to the north, with the exception of a short 

 distance in Sikkim, a zone of rocks of Tertiary age separates the 

 alluvium on the one hand from the pre-Tertiary rocks of the 

 Himalayas on the other. In a word, the southern boundary is due 

 to the freaks of denudation and alteration of level, while the 

 northern is a structural boundary ; and this will be made still 

 more evident when we come to consider in more detail that band 

 of Tertiary deposits which forms the southern margin of the 

 Himalayas, and has so important a bearing on the subject with, 

 which this paper deals. 



Part I. The Facts. 



The Tertiaries of the outer Himalayas present a marked similarity 

 of type along the whole extent of the range, but the sequence is 

 not everywhere equally complete. In the extreme north-west there 

 is a great series of beds, ranging from the marine Nummulitics at 

 the base to the subaerially deposited Pliocene conglomerate sand- 

 stones and clays at the summit. They were divided by Mr. Wynne, 

 who examined this country, as follows : ^ — 



1. Upper Siwalik, about 4000 feet; brown drab or reddish clays, soft grey sand- 

 stones, conglomerates. 



2. Lower Siwalik, about 10,000 feet; soft grey sandstones and brown and grey 

 clays, many red clays. 



3. Murree beds, average 7500 feet ; harder grey sandstones with soft zones, red 

 or purple clays. 



4. Nummulitic, upper 800 feet ; greenish grey and purple sandstones, grey, olive- 

 brownish, red and variegated clays with masses of gypsum. 



5. JSTummulitic limestones with shales and coal at "base. 



The series is said to be parallel and conformable from the pale 

 limestones upwards to the top of the Siwaliks. 



The series in the Jamu^ (Jummoo) hills appears to be practically 

 the same, except that the Nummulitic Limestone is much less 

 developed. 



In the Simla region the Tertiaries are divided into two distinct 

 areas, in which the beds are of different ages, though they may 

 slightly overlap each other. The first area forms part of the lower 

 Himalayas and the beds constitute the so-called Sirmur series, 

 divided into 



1. The Kasaoli group ; grey sandstones, sandy shales, and shales containing plant- 

 remains. 



2. The Dagshai group ; hard, fine-grained sandstones and red or purple nodular 

 shales, the former prevailing in the upper and the latter in the lower part of the 

 group. 



1 Records Geol. Surv. India, vol. x. p. 112 (1877). 



2 Mem. Geol. Surv. India, vol. xxii. chap. v. (1883). 



