THE BLACKSTONE SEEIES. 105 



well be termed a batholitic complex. Notwithstanding the irregular dis- 

 position of the detached masses of strata, there is traceable a dominant 

 structural system in this belt which is nearly east and west in its trends. 

 The strikes of the strata in the Blackstone Valley are NW.-SE.; those 

 in the area west of Providence, nearly E.— W. Members apparently of an 

 equally ancient system of rocks occur on the southeast of the basin in the 

 gneisses and schists of the New Bedford area (see fig. 7, p. 121), and in 

 small masses near Canton Junction. 



The geological relations of the Blackstone series are not exactly 

 defined along the northern and western border. The lowest stratified 

 member along this border of the basin has not been seen, in the area under 

 discussion, to rest on the necessarily older land mass from which it was 

 derived. This separation is probably due to the intrusion of granitic 

 masses. 



The determination of the pre-Cambrian age of the group of limestones, 

 schists, slates, and quartzites, in the Blackstone River area, rests upon the 

 relation which it bears to the lower Cambrian strata in North Attleboro. 

 The Olenellus fauna occurs in little-altered, red, calcareous shales and slates 

 at this latter place in close proximity to granite (hornblendic granitite). 

 Four miles Avest of this inlier of the Carboniferous area occur the sedi- 

 ments involved in the complex already described. These strata are highly 

 altered sediments, now hornblendic and chloritic schists, mainly of a green 

 color, altered sandstones or quartzites, and crystalline limestones. The 

 presumption that these rocks are pre-Cambrian rests, at present, therefore, 

 on the difference in metamorphism between them and the lower Cambrian 

 rocks in the same field. The criterion appealed to in this case is embodied 

 in the statement that where two sets of rocks coexist in the same dynamic 

 field, that group which has undergone one dynamic movement more than 

 the other is the older. If this view is maintained, this series of rocks falls 

 into the Algonkian. Evidence of unconformity with the lower Cambrian 

 is necessary to make this conclusion positive. The relation of the granitic 

 intrusives to the pre-Cambrian on the one hand and to the Cambrian on the 

 other is simply to show that the granitite is younger than the former, and 

 that the sedimentary rocks are of different ages. 



From the typical development of this series in the lower course of the 

 Blackstone River between Woonsocket and Pawtucket, it is here proposed 



