112 GEOLOGY OF THE NARRAGANSETT BASIK 



phitic crusts, and less commonly as white calcareous shells. The shell ; occur mostly 

 as detached valves, with their concave or interior faces turned downward upon the 

 bedding planes, evidently as the result of current action strong enough to invert the 

 shells when their saucer-shaped edges were opposed to the bottom drift, but not 

 ' powerful enough to move them when their smooth oval backs were presented to the 

 moving water. The existence of currents and shallow-water conditions is further 

 attested by examples of pebbles with marked cross bedding, the "top-set" layers of 

 which, with contained shells, confirm the explanation here given for the position of 

 detached brachiopod valves. 



The variations in the quartzites and the irregularities in the fossiliferous layers 

 suggest that this zone alternated with bands of barren quartzite, described as No. 3. 



2. A Scolithus linearis zone of light-colored quartzites. — Pebbles carrying Scolithus 

 are by no means so common as the Obolus pebbles; they are most abundant in the 

 beaches of Marthas Vineyard. A cobble found on the beach of Marsh field Neck was 

 10i inches long, 6 inches thick, and 6 inches across, the burrows being 6 inches long 

 and worn off at both ends. Walcott notes their absence from the materials which he 

 studied. 



3. A barren zone or zones of quartzites of various colors. — Some of the quartzite 

 pebbles included under this head may be of other than upper Cambrian age. since 

 quartzites of different periods occur along the western margin, of the basin. 



A few quartzite pebbles traversed with quartz veins older than the 

 Carboniferous sediments have been observed, from which it is to be inferred 

 that the upper Cambrian formation underwent some deformation attended 

 by the segregation of silica in veins while the beds were still deeply- 

 buried, but under what depth of cover is not exactly known. The presence 

 of Silurian fossils in the Miocene gravels of Marthas Vineyard may indicate 

 the continuance of deposition in this held during the succeeding Silurian 

 period. . 



Until fossils are found in the quartzites which are known to occur in 

 the outlying- region, the exact source of the pebbles in the Carboniferous 

 conglomerates must remain in doubt. It should be noted that the erosion 

 of pre-Carboniferous quartzites has furnished at least three-fourths of the 

 coarse fragmental material in the Carboniferous grits and conglomerates. 

 The area of erosion of so much coarse material, ranging to pebbles a foot 

 or more in diameter, as must have been the case with the now elongated 

 quartzite pebbles of the conglomerate at Newport, could not have been far 

 distant. 



While the preponderance of large quartzite pebbles and fossiliferous 

 examples in the southern portion of the Carboniferous field favors the 



