132 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



In 1899 a preliminary study was made by the writer 3 of the effusives of 

 this basin. Since that time, owing to tunnelling and excavations, and through 

 the invaluable assistance and courtesy of Professor W. O. Crosby, it has been 

 possible to secure further material and to gather data on the hitherto obscure 

 relations of the lava flows, the dikes, and the deep-seated granolitic rocks. It is 

 the purpose of this paper to present these relations and to show how far they are 

 sustained by the petrographic and chemical character of the igneous types. 



The writer finds it a pleasure to express her profound obligations to Professor 

 W. 0. Crosby, at whose suggestion this study was undertaken, whose co-operation 

 has rendered the investigation possible, and whose unequalled familiarity with 

 this district has been at the service of the writer. Professor Crosby has afforded 

 the writer opportunities to visit the district and to collect material and has 

 liberally supplied her with excellent material collected by himself. The nine 

 chemical analyses which appear in this paper were secured through Professor 



Crosby. 



GENERAL GEOLOGY OF THE VALLEY. 



The general geology of the region has been investigated by Professor Crosby 4 

 and is aside from the purpose of this paper. A brief statement of the strati- 

 graphic and structural features of the valley will be pertinent in their bearing 

 upon the age of the igneous material. 



The prevailing rock is a conglomerate and the prevailing structure is anti- 

 clinal. Along the northern margin of the valley the conglomerate dips to the 

 north under a slate and along the southern margin the dips are to the south and 

 the conglomerate passes again under the slate; this structure is complicated 

 by several sharp synclines and numerous faults. In the central and western 

 part of the valley erosion has uncovered the igneous rocks : the acid volcanics and 

 plutonics are the floor upon which the conglomerate rests, while the more basic 

 volcanics occur as intrusives and as flows interbedded with the conglomerate, 

 there are three flows of non-porphyritic basic lava and one of a porphyry of inter- 

 mediate composition. The conglomerate is of Carboniferous age; the acid ig- 

 neous material is then pre-Carboniferous and the basic igneous material Car- 

 boniferous. 



The igneous rocks are exposed in four considerable areas with many small ou - 

 lying bodies. The relations of the acid igneous types are best seen in the Stony 

 Brook Reservation and adjacent territory, which, in a general way, is a repetition 

 on a small scale of the Blue Hills Complex. The oldest formation exposed is a 

 normal, medium to coarse-grained, arfvedsonrte granite; this granite undoubte y 

 constitutes the main body of a batholite formed in and beneath Cambrian strata, 

 of which a few remnants remain uneroded. The contact zone developed on 

 periphery of this batholite consists, as in the Blue Hills, in part of a fine-graine 



8 Volcanics of Neponset Valley, Bulletin of the Geol. Soc. Amer., vol. II, pp. 115~ 126 ' 19 ?^ setts . 

 • W. O. Crosby, Genetic and Structural Relations of the Lower Neponset Valley, Massac_ n 

 Amer. Geol., vol. 36, 1905, pp. 34-47, 69-83. Tech. Quart., vol. XVIII, no. 4, 1905, PP- **>-* 



