MUSEUM OF COMPARATIVE ZOOLOGY. AT 
mass is not more altered than is usual with our unchanged conglomer- 
ates. Its general character, save for its reddish color, is undistinguish- 
able from the deposits of Millstone Grit age in the neighboring coal-tield. 
This wide difference in the measure of metamorphism of the rocks which 
are traced almost in contact with each other is of itself sufficient evi- 
dence of great disparity in age. ‘Too much importance, however, should 
not be given to this consideration, for the reason, as I shall have here- 
after occasion to show in the final report on the Narragansett field, that 
the Carboniferous rocks in the southern portion of the basin appear to 
have undergone a very extensive regional metamorphism in which the 
sandstones have taken on a gneissic shape, the black shales been con- 
verted into otrolitic schists, and the conglomerates also metamorphosed, 
the cement taking on a gneissoid form, and the composition in many of 
the pebbles being similarly changed. This alteration, however, seems 
to come about gradually as we go from the north southward, while the 
corresponding change in passing from the Cambrian to the subjacent 
rocks is of a sudden nature. 
The rocks of apparently Pre-Cambrian age, possibly to be assigned to 
the Huronian period, which lie to the west of the Cambrian field, differ 
in their attitudes from those of the Carboniferous as well as the Cam- 
brian series. The prevailing strikes in the valley of the Blackstone are 
from northwest to southeast. There are some local variations which 
give other directions, but there can be no question that, considered as a 
field, the highly tilted rocks pretty regularly extend in a northwest and 
southeast direction. Thus the limestone belt which extends from near 
Valley Falls to Harris’s Quarry, about four miles west of that point, 
has a tolerably uniform trend in the above-mentioned direction. At 
the Dexter Quarry they are locally thrown from the prevailing strike, so 
that the axis is nearly north and south; but the general direction of the 
limestone belt is nearly that above described. On the other hand, the 
rocks of the Cambrian, as well as those of the Carboniferous, have a toler- 
ably uniform northeast and southwest trend, the strikes varying from 
north to north 45° east, thus following the general course of the disloca- 
tions along the Atlantic coast. It therefore appears that there must 
have been a change in the character of the tension and consequent dis- 
ruption which have affected this country in Pre- and Post-Cambrian 
times, the more ancient rocks having undergone extensive displacements 
in a peculiar axis before the later deposits were accumulated. 
In this connection it may be interesting to note that the beds of 
Tertiary age on Martha’s Vineyard, about fifty miles to the southeast of 
VOL. XVI. — NO. 2. 2 
