MANSFIELD: ROXBURY CONGLOMERATE. 149 
phous carbonate of lime and not of organic remains (Strahan, p. 143- 
144). Current markings, sun-cracks and footprints or other impres- 
sions common on exposed mud-flats, are frequent in estuarine and 
perhaps in fluviatile or lacustrine deposits but do not ordinarily occur 
in marine formations. 
In crush-conglomerates no true bedding appears and all traces of 
the original bedding may have been destroyed. The bedding of ice- 
laid deposits is very obscure and that of fluvio-glacial deposits merges 
into that of true fluviatile deposits so that little or no distinction can 
be drawn. In summarizing the discussion of bedding it may be said 
that marine deposits give on the whole the most even and regular 
stratification, while fluviatile accumulations present the greatest vari- 
ety. Ice-laid materials and crush-conglomerates show little or no 
bedding. 
Relations to Subjacent Rocks. The main fact brought out by the 
investigation of the relations of conglomerates to subjacent rocks is 
that those formations of any age that have been proved to be glacial 
have been found to rest upon striated rock surfaces. The possession 
of heterogeneous structure, irregular and striated pebbles, while fur- 
nishing strong evidence of glacial action, cannot be considered as 
conclusive proof, for such structures and forms may be produced in 
other ways. When, however, such forms are found to rest upon a 
smoothly polished and striated rock surface, the weight of evidence 
is so great that no other explanation can be accepted. 
Summary.— Marine sediments exhibit, on the whole, the greatest 
uniformity of composition and the most orderly arrangement of 
materials, while glacial deposits display the opposite characteristics. 
Lacustrine, estuarine, and fluviatile accumulations attain interme- 
diate degrees of uniformity. Marine action tends to produce sheets 
of relatively uniform thickness over wide areas, while fluviatile action 
tends to produce interwoven linear bundles of coarser and finer mate- 
rials, which may attain great thickness in the aggregate over limited 
areas, but which thin out more rapidly than is the case with marine 
deposits. 
Each of the various types of conglomerate possesses features that are 
shared to some extent by other types. Thus there is no single feature 
which in itself distinguishes any particular kind of conglomerate. 
It is only when a number of features of one type are grouped and 
‚compared with a similar group of another type that definite distinc- 
tions can be made. Such a comparison is attempted in the accom- 
panying tabular summary: — 
