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MANSFIELD: ROXBURY CONGLOMERATE. 258 
evenly bedded than is usually the case with marine sediments, and 
that contemporaneous erosion has occurred in all three basins. More- 
over, the fossils that have been found in the Norfolk and Nar ragansett 
Basins are more closely allied to fresh-water than to marine types. 
Thus it appears that in the Narragansett and Norfolk Basins, at 
least, the sea was not the agent that finally deposited the sediments. 
In the Boston Basin the lithological similarity of the Roxbury Conglom- 
erate to the Norfolk and Narragansett Basin Conglomerates, together 
with the resemblance of the obscure fossils discovered by Burr to 
Certain plant remains discovered by Crosby and Barton in the Nor- 
folk Basin, favor a like conclusion. On the other hand, the Somer- 
ville and Neponset slates are unlike the slates and shales of the 
Narragansett Basin. The latter are often micaceous, carbonaceous, 
and fossiliferous and more or less intermingled with sand. The 
Somerville and Neponset slates, on the other hand, are more uniform 
in texture, not micaceous and not certainly fossiliferous, though 
obscure traces of fossils have been found that tend to ally the slates 
more with «marine than with fresh-water conditions. As stated 
previously, these differences form the main basis of the argument 
for the Cambrian age of these slates. Assuming them to be Car- 
boniferous, however, as their structure seems to indicate, it seems 
hardly possible to consider them marine in view of the probable 
fresh-water origin of the great mass of sediments in the three basins, 
with which they appear to be intimately associated. 
Lacustrine. The idea of the existence of lakes during the deposi- 
tion of the Carboniferous sediments of the Narragansett Basin is sug- 
gested by both Shaler and Woodworth (Shaler et al., p. 53, 177). 
he comparisons on pages 248 and 249 have shown that the texture, 
character, and bedding of the several members of the series in the three 
basins more closely resemble the characteristics of lacustrine sediments 
than of marine deposits; for, while a certain degree of uniformity and 
regularity in composition and stratification is attained by the Car- 
boniferous deposits, these features are not so well developed as might 
© expected in the case of marine strata. The apparent gradation in 
texture in the sediments of the Boston Basin tends to favor the marine 
idea but it has been shown that upper and perhaps coarser members of 
the series have probably been eroded away. The evidences of fresh- 
Water origin cited in the previous paragraphs favor the hypothesis of 
acustrine origin for at least the more uniformly arranged portions of 
the sediments. On the other hand, the coarseness of the conglom-) 
‘rate and the irregularities of texture and bedding of certain parts 
