UPPER CAMBRIAN PEBBLES. 113 



idea that the derivation, was from the south and east, the evidence is 

 good only for the pebbles in that part of the area. The quartz pebbles on 

 the noi-th and west may equally well have been derived from those 

 directions for all that is known regarding the neighboring areas, particu- 

 larly since the texture of the Carboniferous deposits is mainly consonant 

 with tlie hypothesis of a peripheral origin of the sediments contained 

 within the present limits of the basin. 



In conclusion, it need only be said that there appears to have been 

 nearly continuous deposition in this field throughout the Cambrian period, 

 for though the middle Cambrian has not been identified south of Braintree, 

 it hes within the same geological province. The change from the mud of 

 middle Cambrian times to sandy bottoms of npper Cambrian times in this 

 portion of New England apparently indicates progressive shoaling and 

 uplift of the sea floor. 



Since the above account was written, Walcott^ has published a careful 

 revision of the fauna of these fossiliferous pebbles, referring the forms to 

 Obolus (Lingulohulus) affinis Billings and 0. (£.) spissm Billings, and to a 

 new species Obolus {Lingulelld) rogersL Walcott suggests the derivation of 

 the fragments from an area of erosion lying in the vicinity of Newfoundland. 



SIIiURIAK PERIOD. 



There are no known clastic rocks of Silurian age in situ in this basin. 

 Certain terranes in the southern and western parts of the area formerly 

 referred to this period are now regarded as either Cambrian or Carboniferous. 



OJtxiitXbX irJJjJtSJSXiiliD* 



The possible former ^existence of a Silurian formation in this portion 

 of the coast is indicated by the occurrence of chert pebbles carrying 

 upper Silurian corals in the Miocene gravels^ — the '* osseous conglomerate" 

 of Hitchcock — at Gay Head and elsewhere on Marthaa Vineyard, I am 

 indebted to Mr. C. D. Walcott for the determination of the age of these 

 chert pebbles.^ Their origin is involved in the same mystery which sur- 

 rounds the upper Cambrian pebbles. 



1 Note on tlie braohiopod fanna of the qnartzitic pebbles of the Carboniferous conglomerates ol' 

 the Narragandett Basin, Rhode Island, by C- B. Waloott: Am. Jour. Soi., 4th series, Vol. VI, 1S% pp. 



327-528. 



^In an early notice of them, based on scanty material, I mistook the single coral then found for 

 a Cambrian form. Am. Geologist, Vol. IX, 1893, pp. 243-247. 



MON xxxni— ^S 



