It. D. Oldham — Essays in Theoretical Geology. 9 



throughout the greater part of the period which has elapsed since 

 the commencement of the sedimentaiy period. The region seems 

 to have undergone but little oscillation of level, and such oscillations 

 as have taken place never brought more than the margins of the area 

 beneath the level of the sea. With the exception of the Aravallis 

 before mentioned, there is no structural mountain-range in the 

 Peninsular area, and such tracts as, from their elevation, deserve the 

 name of mountains, owe their limitations to the action of subaerial 

 denudation. 



In conti'ast with this the Extra-peninsular area, if we except the 

 Upper Tertiary beds at the top of the sequence and the Archaean 

 gneisses at the base, contains few sedimentary rocks which are not 

 of marine origin. Almost everywhere they are intensely disturbed, 

 and the ai'ea is essentially one of structui'al mountain ranges, that is, 

 of ranges whose general direction is closely connected with, and 

 caused by, the disturbance the beds have undergone. Of this area 

 we are at present concerned with the Himalayas, that system of 

 mountains which rises abruptly from the plains of Upper India, 

 range beyond range, to the snow-clad summits forming the highest 

 spots on the surface of the Earth. 



In the contour of the two areas there is a vast difference. The 

 surface of the Peninsular area shows rounded outlying and gentle 

 slopes, except where the outcrop of hard or massive beds has led 

 to the formation of scarps ; the gradients of the streams are flat, their 

 beds sandy, and seldom do they show signs of active erosion of 

 their channels ; in fact, the Peninsular area shows all the features 

 of an ancient land-surface, long exposed to subaerial denudation, 

 in which the streams and hill-slopes have alike gone far towards 

 reaching a condition of equilibrium. 



The Himalayas, on the contrary, are a region of deep, steep-sided, 

 valleys and gorges; nearly everywhere the streams and rivers alike 

 are torrents flowing over a bed of coarse boulders, everywhere, with 

 but local exceptions, they are evidently eroding their channels, 

 while the steep slopes and sharp crests of the ridges, no less than 

 the constant landslips and general instability of the soil, betoken 

 a country in which the action of rain and frosts on the hill-sides has 

 not been able to keep pace with the deepening of the stream-beds 

 by erosion. 



Equally striking is the contrast in the nature of the boundary of 

 the two areas with the Tndo-gangetic alluvium. To the north the 

 Himalayas rise abruptly from slopes of recent gravels at their base, 

 and, though there are irregularities of detail, the boundary as a 

 whole sweeps in a crescent curve from one extremity to the other, 

 while there are no outliers of the Himalayan beds surrounded by 

 alluvium, nor are there any long tongues of the latter running up 

 into the Himalayan area. To the south the rocks slip imperceptibly 

 under the alluvium, so much so that different observers have differed 

 by miles in the boundar}' they drew between the alluvium and the 

 older rock ; the boundary exhibits every variety of irregularity, 

 long tongues of alluvium run up the various river valleys far into 



