MANSFIELD: ROXBURY CONGLOMERATE. 147 
senting the most nearly perfect, and glacial deposits the least perfect, 
assortment and arrangement of particles, the intermediate stages 
being lacustrine, estuarine, fluviatile, and crush. 
Pebbles. The preceding accounts have shown that the pebbles of 
marine and lacustrine conglomerates tend to be well sorted and well 
rounded, though they may be subangular in proximity to their sources. 
Shrubsole, noting the way in which pebbles slip over each other with 
the recession of each wave, remarks, “the pebbles become as a rule 
symmetrical and lose all traces of angularity” (Shrubsole, p. 315). 
Estuarine pebbles tend to be but imperfectly sorted and rounded and 
fluviatile pebbles may show all stages from confused heaps to well- 
Stratified beds and from well-rounded forms to almost complete 
angularity. The difference between marine and fluviatile pebbles 
is thus expressed by Dutton: ‘‘Attrition” (in the fluviatile con- 
glomerates of the High Plateaus) “is not ordinarily extreme. In 
most cases it is enough to indicate that the fragments are really 
abraded, though with no great loss of substance. The stones of 
sub-aqueous conglomerates, on the contrary, are almost always much 
Worn and rounded. Again, the sizes of the stones” (in the flu- 
viatile conglomerate) “range from a fraction of a cubic inch to sev- 
eral cubic feet; in rare instances to more than a cubic yard” (Dutton, 
p. 224). In crush-conglomerates the shapes and sizes are variable 
depending on the character of the rocks crushed and on the character 
and amount of the deforming force. No doubt the pebbles would 
often be distorted and contain fracture planes and tension cracks. 
Glacial pebbles are characterized by variety in composition, size, and 
shape. Their sizes and shapes may, however, be so successfully 
imitated by pebbles and boulders of fluviatile origin that it is only 
when the fragments are seen to bear the characteristic glacial striae 
or to be intimately associated with stones that are so marked that their 
glacial nature can be regarded as established. Even here caution is 
needed, for in land slides or mud flows striated pebbles may be pro- 
duced, which closely resemble those developed by glacial action. In 
this connection it is worth while to note the view of Meunier, who 
Suggested that even such extensive accumulations as the Dwyka Con- 
glomerate might be the result of mud flows in which later movements 
had been induced by the action of percolating waters in removing 
finer material (Meunier, p. 121). Itis probable then that the following 
order represents the relative regularity in size, shape, and arrangement 
of materials in the various types of conglomerate, though here too no 
sharply defined lines of separation can be drawn: marine, lacustrine, 
estuarine, fluviatile or crush, glacial. 
