470 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



Sayles and La Forge ^"^ have published a note on faceted and striated pebbles 

 from the Roxbury conglomerate, from which they conclude that the conglomerate 

 is in part at least of glacial origin. They say: 



The tillite occupies the upper portion of what has been known as the Roxbury conglom- 

 erate and at Squantum is from 500 to 600 feet thick and is overlain by about 60 feet of stratified 

 conglomerate, sandstone, and interbedded slate, which make the top of the Roxbury and 

 form a 'transition to the overlying Cambridge slate. The possible glacial origin of the Roxbury 

 conglomerate has been suggested by the late N. S. Shaler and others, but these are the first 

 known discoveries in that formation of definite evidence of glacial action or of the existence 

 of glacially deposited beds. 



They conclude that the Roxbury conglomerate and Cambridge slate are 

 probably of Permian age. 



The Boston Basin of eastern Massachusetts contains terranes which have 

 been assigned to the Carboniferous, in part on physical and in part on paleontologic 

 evidence, and which consist of arkose, conglomerate, sandstone, and slate. The 

 rocks were described by Crosby ^^^ and have been made the subject of a very detailed 

 study by Mansfield,"" who has given special attention to the origin and distin- 

 guishing characteristics of the conglomerates. He describes them under the head- 

 ings "Roxbury series," "Norfolk Basin series," " Narragansett Basin series," and 

 "Harvard conglomerate." These distinctions are areal rather than stratigraphic 

 and are made because in the absence of fossils and under the conditions of occur- 

 rence exact stratigraphic correlation among the small areas of outcrop was not 

 practicable. Mansfield ""^ concludes that — 



The evidence adduced in the preceding paragraphs as to origin is largely negative and 

 unsatisfactory. The bedding and texture of the sediments, though attaining a fair degree 

 of regularity, do not display these features in so high a degree of development as might be 

 expected in true marine strata. The apparent gradation upward from coarse to fine texture 

 in the Boston Basin, suggestive of marine transgression, is offset by the occurrence in the 

 Narragansett Basin of a gradation in the opposite direction, suggestive of no n marine deposi- 

 tion. The fossils thus far found in the Narragansett and Norfolk basins are indicative of 

 nonmarine rather than of marine origin. Similar evidence is borne by the irregularities of 

 bedding and of texture. The hthological similarity of the Roxbury conglomerate to the 

 rocks of the Norfolk and Narragansett basins makes it probable that all are of like origin 

 and that the entire Carboniferous series of this region is nonmarine. More than one process, 

 however, was concerned in the formation of the series. The more regular and even-bedded 

 portions are suggestive of quiet fluviatile or lacustrine origin, while the more irregular and 

 tumultuous portions of the deposit indicate torrent action. The great quantity of large pebbles 

 of relatively fresh granite and the abundance of feldspathic material in the sandstones and 

 in the matrices of the conglomerates suggest that much material was furnished to the streams 

 of that time by glaciers of which no direct evidence now exists. 



In describing the geology of Worcester and vicinity. Perry and Emerson,®^* 

 after giving an account of local occurrences, summarize the facts relating to the 

 Carboniferous as follows : 



Starting at Worcester, as a center, we find first and uppermost the Worcester phyllite. 

 This rock underlies the central part of the city and may be traced southwesterly and north- 

 easterly across the State. It presents varying stages of metamorphism, from a true argiUite 

 to a well-defined mica schist, and contains in different places minerals resulting from meta- 

 morphism, as garnets, chiastoHte, graphite, stauroUte, and anthracite. The laminae of the 

 phyllite are frequently liigbiy crumpled, producing folds almost infinitesimal in size and infinite 



