MANSFIELD: ROXBURY CONGLOMERATE. T21 
Estuarine Deposits. Not many accounts of estuarine deposits 
were found by the writer. Sollas, in a study of the estuaries of the 
Severn and its tributaries, accounts for the muddiness of the Severn 
estuary by the ineffectiveness of tidal erosion. He quotes (Sollas, 
p. 612-613) the experimental results of W. R. Brown, who found that 
for two-thirds of the ebb, though the surface water runs out rapidly, 
the water at the bottom is practically at rest; only during the remaining 
third of the ebb does the bottom water flow outwards with sufficient 
velocity to scour the channel; this, moreover, lasts so short a time that 
hardly as much mud and sand are removed as have been laid down dur- 
ing the flood and the earlier part of ebb tide. Hence the sediment is in 
continual oscillation up and down the estuary (A. Geikie, p. 510-511). 
Sollas describes the mud, discusses its source and concludes that it is 
largely marine on account of the organic remains. The mud contains 
(Sollas, p. 613) argillaceous granules, small angular fragments of 
quartz containing cavities, a few similar fragments of flint, silicious 
fragments of a glauconitie color, minute crystals of quartz and tour- 
maline, together with sponge spicules, and minute organisms. Sections 
of the alluvial flats (ibid., p. 622) show gravel at the bottom, containing 
rolled pebbles, angular and subangular blocks of Millstone Grit, and 
vein quartz. Some blocks are more than one cubic foot in size and 
one subangular fragment is well smoothed and striated as if by ice. 
Sands, clays, and peat also occur in the section but the sands (ibid., 
p. 623) are not a constant feature; they are either marine or tidal 
and are probably explained by local current action. Sollas does not 
account for the presence of the gravel. 
Willis, discussing the Devonian sediments of Pennsylvania and 
Maryland, remarks: “The unassorted mingling of sandy and clayey 
particles is the result of rapid deposition at the mouths of muddy 
streams in opposition to waves that are too weak to sort and distribute 
the volume of sediment. This is a condition of delta building. The 
frequent and irregular interbedding of coarse sands, sandy clays and 
clays, the cross-stratified beds, the ripple-marked and sun-cracked 
mud surfaces, the channels scoured by transitory streams, all prove 
the abundance of sediments, the shifting conditions of deposit, the 
irregularity of currents, the wide expanse of tide-flats and shallow 
waters and the weakness of the waves” (Willis, c, p. 63). 
Russell (b, p. 46-47) outlines the argument for the estuarine and 
swamp origin of the Newark system as follows: (1) the absence of 
marine or fresh-water fossils suggests brackish water and unstable 
conditions; (2) the fossil fishes are closely allied to the existing ganoids 
