234 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
On the eastern side of the basin, south of Tiverton, a high degree 
of metamorphism has been attained. At Fogland Point the con- 
glomerate pebbles have been. elongated, flattened, indented, and 
striated under pressure, and there has been an extensive development 
of mica. The shape of the pebbles indicates that they must have 
been more or less plastic when deformed, yet the groups of tension 
cracks, some of them passing entirely through the pebbles, indicate 
that the pressure under which deformation took place was not suffi- 
cient to induce perfect plasticity. Plate 5 shows typical ledges of 
the metamorphosed conglomerate. The Purgatory Conglomerate, 
already mentioned (pages 165 and 168), farther south, shows similar 
characteristics, but there the pebbles are much larger. In both these 
localities the elongation of the pebbles must equal or exceed 50 per 
cent of the original diameter on the given axis, as stated by Shaler 
(p. 17). At Purgatory, near Newport, the writer observed a 
pebble or boulder in the conglomerate that measured nine feet in 
length and three feet in width, but some of these boulders are described 
by Hitchcock as attaining a length of twelve feet (C. H. Hitchcock, 
a, p. 113-114). 
Farther within the basin the rocks have been only slightly affected. 
In a quarry half a mile south of the intersection of Thatcher and 
County Roads at Attleboro, opened in the rocks immediately below 
the Dighton conglomerate near the top of the syncline, ripple-marks, 
rain-imprints, and worm-trails appear on the surface of shaly beds 
that are now standing nearly vertical, Woodworth, speaking of 
these features, remarks that the “preservation of this record at this 
locality, where the beds are now vertical, indicates that locally, at 
least, metamorphism in the Rhode Island coal field has not gone so far 
as is commonly believed. ‘The condition of the imprints shows that 
in the folding of the strata on this horizon, at least, there was no wide- 
spread shearing of layer over layer, which in other localities is usually 
marked by slickensides or the appearance of ‘grain’” (d, p. 178). 
Summary. (1) The Narragansett Basin sediments occupy a some- 
what irregular trough about fifty miles long and thirty miles wide, 
rudely concave toward the southeast. 
(2) The boundaries are not determined in the main by faults but 
are usually sedimentary contacts with the surrounding rocks. 
(3) The sedimentary series consists of arkoses, conglomerates, 
sandstones, and fossiliferous shales, with some coal beds. 
(4) The arkoses form the basal members and the highest beds are 
heavy conglomerates. 
