554 GEORGE R. MANSFIELD 
while fluviatile formations may be well stratified, or, on the other hand, 
may so closely simulate heterogeneous glacial accumulations as to 
make their origin uncertain: witness the discussion of the Midland 
Pebble Beds of the Old Red Sandstone. Cross-stratification and 
lenticular masses of coarser and finer material are common in all 
these types, but in the marine type the long axes of the lenses are more 
frequently parallel to the shore line—that is, to the original strike of 
the rocks; while in the case of fluviatile accumulations the long axes 
of the lenses are parallel to the courses of the stream threads by which 
they were deposited—that is, to the original dip of the rocks. All 
water-laid deposits appear to increase in thickness and coarseness 
toward their source of supply. Marine action, however, tends to 
produce sheets of relatively uniform thickness over wide areas, while 
fluviatile action tends to produce interwoven linear bundles of coarser 
and finer materials, which may attain great thickness in the aggre- 
gate over limited areas, but which thin out more rapidly than is the 
case with marine deposits. Other differences are cited by Strahan. 
in his discussion of continental deposits. He states that the latter 
are not only unequal, but alternate with erosion, so that fragments 
of one bed are included as pebbles in another; that they rarely con- 
tain marine organisms or such strata as usually compose marine 
formations, but that drifted plant remains are not uncommon, and 
that such limestones as occur consist, when unaltered, of amorphous 
carbonate of lime and not of organic remains.'. Current markings, 
sun-cracks, and foot-prints, or other impressions common on exposed 
mud-flats, are frequent in estuarine and probably 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. 
As regards the relations of conglomerates to subjacent rocks, the 
main fact brought out by the investigation 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 
1 Loc. cit. 
