PRE-CAMBKIAN ROCKS 445 



ites of Berkshire, which are obviously too far away to enter into the ques- 

 tion. They also compare the former with pre-Carboniferous schist at 

 the southern end of Conanicut Island, and with other similar unfossil- 

 iferous rocks near Little Compton and Newport, whose age is unknown. 

 There is nothing conclusive about any of these instances. Therefore the 

 only criterion remaining is that of relative metamorphism. 



In southeastern New England only two periods of mountain-building 

 have been recognized, the pre-Cambrian and Permian. The effects of 

 the latter are apparent throughout the Narragansett Basin and in other 

 rocks in the vicinity. The metamorphism accompanying this erogenic 

 movement has been more intense in the southern part of the Narragan- 

 sett Basin than in the northern part, but no sharp line can be drawn, as 

 Woodworth represented, between the metamorphosed and the unmeta- 

 morphosed portions. ^^ 



In the area considered in this article the metamorphism has affected 

 most severely the part west of a noi*th-south line connecting Sneech Pond 

 with Iron Mine Hill. Thus the granite porphyry on Cumberland Hill, 

 the riebeckite granite at its western edge, the cumberlandite and gabbro 

 near Iron Mine Hill, and the Milf ord granite and Bellingham series west 

 and northwest of the cumberlandite are all more or less sheared, often 

 with a development of a gneissic or schistose structure. The Milford 

 granite and Bellingham series have been sheared the most, as will be dis- 

 cussed later. The main mass of the riebeckite granite and biotite granite 

 east of it show less metamorphism. The Blackstone series, occurring to 

 the north and to the south of the riebeckite granite, and the large body 

 of green schist surrounded by the granite show the effects of an intense 

 and ancient metamorphism accompanied by alteration producing chlorite 

 and epidote in all of the green schists. It is clear that this metamor- 

 phism could not have taken place in the Permian revolution, or the gran- 

 ite would be affected near the schist. Moreover, a half mile north of the 

 last outcrop of this schist north of Grants Mills occur Cambrian fossil- 

 iferous limestone boulders which Foerste considers to be almost in place.^* 

 This limestone is not metamorphosed. 



On the strength of all this evidence, it seems fairly certain that the 

 Blackstone series are pre-Cambrian and were metamorphosed in pre- 

 Cambrian times. 



Cambrian Eocks 

 In his work on the Narragansett Basin, Foerste found some boulders 



13 p. H. Lahee : Am. Jour. Sci., 4th ser., vol. 33, 1912, p. 249 t£. 

 " Op. clt, p. 393. 



