G. F. Loughlin — Quincy Granite. 17 



Aet. III. — Contribution to the Geology of the Boston and 

 Norfolk Basins, Massachusetts. I. The Strtictural Rela- 

 tions between the Quincy Granite and the Adjacent Sedi- 

 mentary Formations /* by G. F. Loughlin. 



Introduction. 



The rocks, both igneous and sedimentary, of eastern Massa- 

 chusetts and Rhode Island offer a host of most interesting 

 but, as yet, unsolved problems in petrology, structural geology, 

 and stratigraphy. One of these problems, of critical import- 

 ance and at the same time unique, is the relation between the 

 Quincy granite mass, just south of Boston, and the adjacent 

 sedimentary formations of the Boston and Norfolk Basins, to 

 the north and south respectively (fig. 1). 



The Quincy granite mass, including the prominent Blue 

 Hills range, forms a topographic as well as geologic barrier 

 between otherwise continuous areas of conglomerate, sandstone, 

 and shale.f The sediments of both basins were believed by 

 Prof. Crosby to be the younger, and to contain pebbles of the 

 Quincy granite. The Quincy granite was believed to be of 

 the same age as all the granites of the district. More recent 

 work by Mansfield^ failed to prove the presence of Quincy 

 granite pebbles in the conglomerate members of the sediments, 

 though pebbles of other granites in the district are plentiful. 

 ISTo explanation of this general absence of Quincy granite peb- 

 bles in the conglomerates is offered, but the sediments are, 

 nevertheless, assumed to be younger than the granite. Clapp§ 

 still more recently has found evidence which shows that the 

 alkaline granites in eastern Massachusetts, of which the Quincy 

 granite is one, are distinctly younger than the other granites 

 and related rocks of the district. 



* The field study of this problem was greatly simplified by the exhaustive 

 work of Prof. W. O. Crosby (see Geol. of Boston Basin, Vol. I, Part III, 

 1900), who has located every outcrop, and discussed at great length the 

 major structural features in the district here considered. The present paper 

 is a modification of certain of his results, based chiefly on the evidence of 

 petrographic study, but is quite in accord with his interpretation of the 

 major structural features of the area. 



f The solitary outcrop of gray quartzitic sandstone due west of Great Blue 

 Hill contains partings of carbonaceous shale, and more closely resembles 

 strata in the western Narragansett Basin, E. I., than rocks in the area here 

 discussed. It is of no definite value as a connecting link between the Boston 

 and Norfolk Basins, and may possibly be unconformably related to the rocks 

 in them. 



X Origin and Structure of the Roxbury Conglomerate ; Bull. Comp. Zool. 

 Harvard Univ., vol. xlix, Geol. Ser. vol. viii, No. 4, pp. 161, 168, 169, 1906. 



§ Manuscript thesis in part fulfillment of requirements for degree of Ph.D., 

 Mass. Inst. Tech., Geol. Dept., 1910. 



Am. Jour. Sci. — Fourth Series, Vol. XXXII, No. 187.— July, 1911. 



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