384 GEOLOGY OF THE NARBAGANSETT BASIN. 



along various parts of the escarpment and thence up the Blackstone Valley 

 as far as Ashton and Manville. 



The question as to the geological position of these quartzites is very 

 important, but at present is without a solution. On lithological grounds 

 alone they might be considered of Cambrian age, but there is little real 

 basis for such a determination. Perhaps the best reason so far known for 

 considering these quartzites as of Cambrian age is the abundant occurrence 

 of Cambrian quartzite pebbles in the Carboniferous conglomerates of the 

 Narragansett Basin. Quartzite pebbles occur at all horizons, from the con- 

 glomerate beds just overlying the basal arkoses to the uppermost layers of 

 the coarse Purgatory conglomerate. The lowest horizon at which these 

 quartzite pebbles contain fossil oboli, however, seems so far to be in the 

 conglomerate beds between the Saunderstown sandstone and Aquidneck 

 shale series on the western side of Prudence, and with less certainty on the 

 western side of Bristol Neck and northwest of Riverside. They occur in 

 far greater abundance in the coarse Purgatory conglomerate, being com- 

 monly found in any considerable exposure where long-continued search 

 has been made for the entire length of Aquidneck Island and the eastern 

 shore of the Sakonnet River. They occur again in the corresponding- 

 coarse conglomerate at Dighton, and southwestward. 



A few of the localities where quartzite pebbles with fossil oboli 

 have been found in situ have been indicated on the accompanying geo- 

 logical map. These fossil oboli belong probably to some late Cambrian 

 horizon. Hence the quartzite deposits elsewhere in the basin, especially 

 those near Natick, in which we are most interested, may also be of late 

 Cambrian age. 



After a careful comparison of the quartzite pebbles in the quartzites at 

 Natick and elsewhere, it must be admitted, however, that the lithological 

 resemblance is not very close. The varied coloring shown by different 

 fossiliferous quartzite pebbles, especially a rather common tint of faintly 

 dark blue, altogether fails in the quartzite exposures so far examined. The 

 whiter more vitreous quartzite localities, moreover, do not present the same 

 cleavage as the whiter fossil-containing pebbles. Furthermore, when the 

 considerable frequence of oboli-containing pebbles is considered, it is rather 

 perplexing to find the quartzite localities in question apparently without 

 fossils. 



