144 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
locally eroded. Ripple-marks may be indicated in some cases by 
cross-bedding, as in the Medina example described by Gilbert,* but 
mud-cracks, footprints and other impressions indicative of former 
extensive mud-flats would not be expected to occur in strictly marine 
deposits. Conglomerates formed by a transgressing sea may be 
expected to maintain a relatively moderate thickness over a wide area, 
as in the Cretaceous basal conglomerate of Texas. The Pottsville 
Conglomerate does not, however, conform to this expectation, for it 
maintains a great thickness in its southeastern portion but thins out 
rapidly northwestward, by the loss of its lower members. Such rapid 
diminution, though strongly contrasted with the Texas example, is 
entirely consistent with the idea of fluviatile origin. 
-—Fluviatile. The criteria given by Oldham in support of the 
fluviatile origin of the Gondwana sediments, summarize much of the 
data cited with regard to the bedding of fluviatile conglomerates. He 
says (a, p. 150-151): “The frequent alternation of coarse and fine 
beds, the frequency of current markings on the finer shales and of 
oblique lamination, due to deposition by a current, in ‚the coarser 
sandstones and the circumstance of the upper portions of a bed, such 
as a coal seam, being locally worn and denuded where a coarse sand- 
stone is deposited upon it, a phenomenon of frequent occurrence, are 
quite consistent with the theory of deposition in a river valley.” 
Some other important features deserve notice. The gravel beds de- 
crease rapidly in thickness laterally away from the streams by which 
they were deposited, as shown by the Siwalik and Bhäbar accumu- 
lations in India, and by the great alluvial cones described by Drew, 
Dutton, and Johnson. Dutton states that in alluvial cones sections 
along the radii give the best stratification but that in transverse sec- 
tions the stratification is less uniform and harmonious. Johnson 
speaks (cf., p. 117) of the great debris slope, of which the present 
High Plains are but a remnant, as composed of “interlaced gravel 
courses penetrating a mass of fine material.” Since deltas are 
essentially prolongations of river flood plains they will preserve in 
the main the characteristics of fluviatile deposits but in consequence 
of their being built forward into a body of water, instead of being 
spread out fan-like on the land, they present the additional feature 
of steeply inclined fore-set beds noted above. 
:—Lacustrine. The bedding of lacustrine sediments will tend 
on the whole to conform more closely with marine than with fluviatile 
1 According to Fairchild the “ giant ripples” of the Medina sandstone represent for- 
mer beach crests. 
