THE BLAOKSTOI^TE SERIES. 107 



exposures at the present surface are due to the truncation of close-pressed 

 folds in which the quartzites occupy anticlinal axes. 



ASHTON SCHISTS. 



Reasons have been advanced above which make it probable that the 

 argillaceous rocks of the Blackstone series ai^e the finer sediments succeed- 

 ing the deposition and partial erosion of the Cumberland quartzites. Dis- 

 tinctions in the several broad bands of these rocks may be based upon the 

 prevalence of siliceous sediment on the one hand and of the chloritic and 

 hornblendic metamorphic products on the other. Not less manifest are 

 differences of secondary structure, on which the slaty and schistose char- 

 acter of the beds in many places depends. All along the border of the 

 area; where the rocks are in contact with granitite and quartz-porphyry, 

 zones of local metamorphism occur in which the characteristic effects of 

 igneous intrusions are to be observed. The series as a whole is character- 

 ized by its greenish color. Some of the rocks included in the schists are 

 probably of igneous origin. 



SMITHFIELD LIMESTONES. 



Owing to the setting off of Lincoln from Smithfield in 1871, the areas 

 of limestone formerly designated by the name here used are no longer in 

 the town of that name. The areas of this rock are isolated, ovoidal in out- 

 line, and have no very systematic distribution. In the main, they lie near 

 granitic masses, as between the valleys of the Blackstone and Moshassuck 

 rivers. The areas are a mile or more apart. Those in the area referred to, 

 including Lime Rock, are in general along the same horizon in the Black- 

 stone series. Most of the areas are in the Blackstone Valley, but there are 

 other outcrops south of the Smithfield granitic mass, in North Providence 

 and Cranston. 



The limestone are finely granular^ dolomitic, crystalline aggregates, 

 and have a laminated structure. Shear zones with development of chloritic 

 minerals are common. The rocks have been subjected to extensive shear- 

 ing and crushing movements, with the consequent faulting of small dikes, 

 as in the Lime Rock quarries. The limestone occupying the space between 

 these dismembered dikes shows to the eye no trace of the separation. The 



