MANSFIELD: ROXBURY CONGLOMERATE. 141 
accumulations. Here again its questionable character as a marine 
conglomerate is displayed. 
:— Fluviatile. The materials of which the pebbles of fluviatile 
conglomerates are composed appear to vary largely with the kinds of 
rock exposed within the drainage areas of the streams and are to a 
considerable extent local in character. In the case of the larger rivers 
the material has usually been carried farther and has been taken from 
hard rocks. The pebbles at any given locality may vary in size from 
fragments hardly larger than grains of sand to masses several feet in 
diameter. The component masses of the conglomerate diminish in size 
in the direction of the grade of the stream that deposited them and 
laterally too they grow smaller away from the larger streams. In 
the portions of the deposit near the source of supply, the growth of the 
accumulations causes the coarser material to advance and overlie finer, 
previously formed detritus, so that coarse conglomerates appear above 
finer sediments. Farther away from the source of supply deep borings 
have been made in fluviatile deposits without showing any notable in- 
crease or decrease in coarseness of material. As to shape, the pebbles 
are variously described as rounded, subangular, and “unrounded.’ In 
Some regions they become nearly, if not quite, angular, while in others 
they approach rotundity. There is thus a lack of uniformity in shape 
as well as in size. In some cases they are described as horizontally 
or obliquely bedded, but in others it is stated that they lie with their 
long axes at all angles with the horizontal plane and without any 
definite arrangement. 
:—Lacustrine. No satisfactory evidence has been found by 
which pebbles formed by lacustrine processes can be distinguished from 
those of other origin. Since the littoral processes of lakes so closely 
resemble those of oceans it is reasonable to believe that lacustrine 
pebbles will resemble more closely those of marine origin than any 
other type. We may expect, therefore, that on the whole lacustrine 
pebbles will be well rounded, but, on account of the absence of tides, 
they may not be so well sorted nor show such general uniformity of 
Size at any given locality as is true of marine pebbles. 
——:—Estuarine. The data presented under the head of estuarine 
deposits show that wave action in estuaries is relatively weak. The 
Coarser material from streams and other sources is therefore subjected 
to little attrition from this cause and is only slowly modified in shape. 
As a result the pebbles may be rolled, subangular, or even angular in 
form. They will, however, tend to be arranged in irregular and cross- 
Stratified deposits by shifting and transitory currents. 
