THE CARBONIFEROUS STRATA. 159 



COAL MEASURES. 



The Rhode Island Coal Measures, if we use this term to comprise all 

 the horizons on which coal has been reported, include at one point or 

 another in the basin all the strata from near the base to the great con- 

 glomerate bed which occurs at the bottom of the Dighton group. The 

 estimated thickness of this section is about 10,000 feet. 



There is reason to believe that the lowest members of this great thick- 

 ness of sediments are, from Pawtucket southward, the time equivalents of 

 the Wamsutta group. The relations of the Wamsutta series of red and 

 green slates to the coal shales may be seen to advantage in the gorge of 

 the Blackstoue River at Pawtucket. 



The very considerable thickness of the beds between the basal arkoses 

 and the conglomerates and the overlying Dighton group of conglomerates 

 has rendered it possible to make certain divisions in this great middle section 

 which have a geographical value and indicate at the same time litholog'ical 

 peculiarities. On these grounds four groups have been denominated, it not 

 being satisfactorily determined whether the strata of the lower two beds 

 are exactly equivalent or not. 



The supposed relations of these to the beds recognized by Dr. Foerste 

 farther south are indicated in the table on page 134. 



In the following notes concerning the northern area local names will 

 be emploved, with such chronological limitations as present knowledge of 

 the field will permit 



CRANSTON BEDS: KINGSTOWN SERIES OF DR. FOERSTE. 



PROVIDENCE AREA. 



It is the general opinion of those who have examined the rocks west 

 of Narragansett Bay in the vicinity of Providence that the strata of the 

 Coal Measures are here nearer the base of the series than those which lie 

 immediately on the east of Providence. This supposition is borne out by 

 the structure of the sections which can be drawn for this region. 



The strata of this area from Pawtucket southward, to the limits of the 

 Providence sheet, exhibit generally very steep dips prevailingly eastward. 

 The strikes are generally east of north, being more persistent in the sand- 



