MANSFIELD: ROXBURY CONGLOMERATE. 443 
years have been shown to possess a greater geological importance than 
was formerly accorded to them. 
Fluviatile Conglomerates: — India. The great Gondwana system 
of India, consisting mainly of sandstones and shales with. some 
coal beds and ranging in age from Upper Carboniferous to Middle 
Mesozoic has been ascribed chiefly to fluviatile action (A. Geikie, 
p. 1058). With the exception of the Talchir group, which will 
be described in a later section, there are no conglomerates of impor- 
tance in the system; but the latter is of interest because it furnishes 
some means whereby fluviatile sediments may be distinguished 
from those of other origin. These will be noted later. 
The Siwalik group, a Tertiary deposit, exposed in the hills of the 
same name along the southern base of the Himalayas, has also been 
ascribed to fluviatile action. The thickness of the series in the hills 
is at least 1,500 feet, while in the northwest Punjab it is estimated at 
14,000 feet (ibid., p. 1207). The group consists of sandstones with 
interbedded clay bands and some coarse conglomerates with well- 
rounded pebbles. Some of the finer beds of the lower part of the sec- 
tion are of bright red or of purplish color. The upper Siwaliks are 
more sandy, only occasionally tinged with red, and toward the top they 
become conglomeratic in bands that increase in abundance and coarse- 
ness. Near the large rivers the uppermost group consists principally 
of coarse conglomerate composed of rounded boulders of hard rock. 
The intermediate stretches are composed largely of soft earthy beds 
precisely similar to the modern alluvium of the plains (Oldham, a, 
p. 356-358). 
The Bhäbar or gravel slope that today fringes the outer margin of 
the hills possesses a remarkable resemblance to the deposits of the 
upper Siwaliks. The extent and composition of the Bhäbar varies 
with its position. The greatest development is opposite the débouchés 
of the great rivers, where the deposits consist almost entirely of boulders 
of hard rocks. In the intervening stretches the composition varies 
with that of the rocks exposed in the drainage areas of the streams. 
The pebbles are often subangular, owing to the shorter distance of 
transportation and less abrasion. ‘They are always less rounded than 
the hard boulders of the great rivers. The upper Siwaliks vary in 
development in similar fashion. Where the Sutlej emerges from the 
hills there are at least 4,000 feet of coarse conglomerates; but in a 
parallel section barely seven miles away, only about 500 feet of con- 
glomerate appear in the midst of over 3,000 feet of sandy clays (ibid., 
p. 469). 
