in the Narragansett Basin. 53 



of bivalve Crustacea of the genera Leaia and Estheria in the 

 u Coal Measures " near Pawtucket, P. I. These are said to 

 correspond closely to specimens found in the "Coal Measures" 

 of Illinois and Indiana, and in the Conemaugh formation of 

 the Carboniferous of Pennsylvania. Haynes remarks that 

 fossils of the two genera occur at several horizons in the " Coal 

 Measures " and that neither genus therefore is a good horizon 

 marker. He also remarks that by correlation with the Cone- 

 maugh formation, this horizon in the Narragansett Basin 

 would be the equivalent of the middle of the " Lower Barren 

 Measures" of middle Pennsylvanian age. There seems, there- 

 fore, from combined fossil evidence, no occasion for doubt that 

 the " Coal Measures " of the JSTarragansett Basin are of 

 Pennsylvanian age. 



The Dighton conglomerate. — The Dighton conglomerate, 

 with which is correlated and mapped the Purgatory con- 

 glomerate of the southeast part of the Basin,* is prevailingly 

 of very coarse texture in marked contrast to the typical " Coal 

 Measures," but includes beds of finer conglomerate and sand- 

 stone. It forms isolated areas of considerable size along 

 synclinal axes (fig. 1) and its beds in some places have very 

 steep dips. The writers have been able to study only the areas 

 in the northern part of the Basin, but find in these a uniform 

 and distinctive composition. The pebbles include, besides an 

 abundance of quartzite, vein quartz, felsite and slate, a large 

 amount of Sterling granite gneiss, and a considerable propor- 

 tion of the metamorphosed " Coal Measures " arkose, of the 

 type identical with that in Foerste's Kingstown series. The 

 Sterling granite pebbles are largely of the muscovitic and peg- 

 matitic contact phase, and also represent the normal gneissoid 

 type. Some of the granite pebbles are free from marked 

 gneissoid structure, and it cannot safely be said whether they 

 represent a eugranitic variation of the Sterling, or a medium 

 grained type of the Milford or Dedham granites. Some 

 of the pebbles are distinctly epidotized, but such an alteration 

 is not a safe criterion for distinction, since the Sterling and 

 Milford granites, as well as the Dedham, are altered locally, 

 and it is very possible that the alteration so characteristic of 

 the Dedham granite was developed during the period of up- 

 heaval which followed deposition of the Dighton conglomerate. 



The identity of the metamorphosed arkose pebbles is proved 

 both by their megascopic and microscopic characters, which 

 agree in all essential details with those of the type rock. One 

 pebble was collected showing an igneous contact between the 

 arkose and the contact phase of Sterling granite gneiss. 



* Mon. TJ. S. Geol. Survey, No. 33, p. 134. 



