MANSFIELD: ROXBURY CONGLOMERATE. 105 
higher power lenses with thin sections. The data thus obtained 
have been compiled and tabulated, together with data derived from 
field observations. 
THE ORIGIN OF CONGLOMERATES. 
GENERAL STATEMENT.— Before taking up the question of the ori- 
gin of the Roxbury Conglomerate it was necessary to find criteria by 
which to judge whether these rocks should be ascribed to marine 
action or to some other mode of formation. An investigation of the 
literature has accordingly been made to determine the characteristics 
of other conglomerates in various parts of the world, ascribed to 
this or that mode of origin. An attempt has been made to examine 
all the more important papers bearing on the subject, but the limita- 
tions of time make it possible that a number of valuable sources of 
information have been overlooked. The present chapter embodies 
citations in which the characteristics of certain types of conglomerate 
are set forth by various writers, together with a discussion and clas- 
sification of the criteria thus obtained. Before entering upon the 
discussion of particular types of conglomerate it is desirable to 
note the characteristics of conglomerate in general and of certain 
allied types of rock. To that end a few definitions and brief de- 
scriptions are given here substantially as in Geikie's Text-book of 
] 
geology. 
DEFINITIONS AND DESCRIPTIONS. —Conglomerates. Rocks formed 
of consolidated gravel or shingle are called conglomerates. The 
component pebbles are rounded and waterworn and may consist 
of any material, though usually of some durable kind, as quartz or 
granite. Different names are given to conglomerates according to 
the character of the pebbles contained, as quartz or granite con- 
glomerates; or according to the nature of the cementing paste or 
matrix, which may consist of hardened sand or clay and may be 
silicious, calcareous, argillaceous, or ferruginous. 
The bedding is not always distinct and it may sometimes be neces- 
sary to view the rock as a whole in its relations to the overlying and 
underlying beds before its stratified character can be conceded (A. 
Geikie, p. 63). The size of the constituent pebbles may range from 
masses several feet in diameter through shingle and gravel of suc- 
cessively finer texture until the rock becomes a grit or a coarse gray- 
