MUSEUM OF COMPARATIVE ZOOLOGY. 17 



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 tha't the beds of 

 Tertiary age on Martha's Vineyard, about fifty miles to the southeast of 



VOL. XVI. — NO. 2. 2 



