MUSEUM OF COMPARATIVE ZOOLOGY. 25 



in the relation of the continent to the sea in this portion of the earth's 

 surface. 



The same evidence which enables us to affirm the frequent presence of 

 the coast line at this point, serves also to indicate that this portion 

 of New England has from a very early date possessed and retained its 

 present mineralogical character. The conglomerates of this Cambrian 

 horizon contain substantially the same kinds of rocks as make up similar 

 detrital deposits of the drift period. So far, I have been unable to dis- 

 cover any varieties of rocks in the one which are not contained in the 

 other, with the single exception of the hornblendic granites, such as 

 are intruded in the form of dikes amid the Cambrian deposits. It 

 appears likely that these materials did not appear in this district until 

 after the Cambrian had been deposited. 



The hornblendic granites which are intruded into the rocks of this 

 Cambrian field have a general likeness to those which appear in the re- 

 gion of the Sharon and Blue Hills. Although in the form of detached 

 masses, they are scattered in a somewhat linear fashion, as in those 

 fields of granite. It is not improbable that their ejection may be of 

 the same date as that of the similar rocks to the northward, but as yet 

 there is no sufficient evidence to make any affirmation in the matter. 



The evidence afforded by the Attleborough series as to the history of 

 the Narragansett Basin, taken in connection with the other facts which 

 I have ascertained in my study of this district, is of a very interesting 

 nature. On the eastern side of the Narragansett synclinal, north of Fall 

 River, the Carboniferous deposits lie immediately upon syenites. On 

 this side of the field the Lower Carboniferous strata are composed, to the 

 thickness of a hundred feet or more, of consolidated waste derived from 

 these crystalline rocks. This waste is so little changed, that at first 

 sight the section appears to be composed of decayed granitic matter. It 

 was only on finding fossils in the deposit at Steep Brook, Mass., where it 

 is quarried for fire-clay, that I became convinced of its Carboniferous 

 age. It thus appears that while on the western part of the basin the 

 Carboniferous series rested upon the great section of Cambrian and 

 Pre-Cambrian rocks, it was bounded on the east by ejections of crystal- 

 line materials. 



This fact enables us in a general way to determiLe something con- 

 cerning the time when these granitic deposits appeared on this part of 

 the continent. They evidently were injected after the formation of the 

 Cambrian, and before the formation of the Carboniferous. At the time 

 when the Coal Measures were deposited these hornblendic granites had 



