204 



structure :iiul physical history of the region and not to under- 

 take a minute investigation of tlie different kinds of rocks. 

 The granitic rocks (dioritc, granite, and felsite) have been 

 ilescribed in sufficient detail in the preceding pages. Mr. 

 Merrill has kindly examined thin sections from some of the 

 more typical dikes, and finds tiiem all to he diabase essentially 

 similar to that of the Nantasket dikes. He has also found that 

 the mela})hyr is in essential agreement with the common basic 

 variety of Nantasket. It has commonly a dark greenish color, 

 due to cidorite and epidote, but limited portions are often 

 brownish or pur[)lish owing to the local peroxidation of the iron. 

 It varies in texture from almost [)erfectly compact or even slaty 

 to distinctly and coarsely amygdaloidal. The amygdules con- 

 sist chiefly of ([uartz and epidote, but include also chlorite, 

 feldspar, and calcite. The quartz is usually crystalline, but 

 sometimes chalcedonic or jaspery. These secondary minerals 

 also occur commonly in veinlets and irregular segregations ; 

 but the brecciation so characteristic of the great bed of melaphyr 

 near the top of the Nantasket series is rarely distinctly observed 

 in Ilingham ; although the scoriaceous structure which one 

 naturally looks for in the superficial portions of a flow is plain 

 enough at some points, as in the mass of melaphyr near the 

 junction of Downer Avenue and Crow Point Lane. The 

 Hingham bed, like the Nantasket bed just referred to, is 

 [)robably coni[)Osite, a succession of flows, but the only t)bserved 

 facts, besides its great thickness, wliich clearly point to this 

 conclusion, are the intercalated bed of sandstone and conglom- 

 erate which is exposed south of Beal Street, about midway 

 between the street and the little pond (PI. 8) ; and a small 

 amount of banded slate or possibly tuff" which a recent excavation 

 has exposed in the melaphyr immediately west of West Hingham 

 Station. The occurrence last mentioned is the only thing 

 resembling volcanic tuft\ or in any way suggestive of exi)losive 

 eruptions, which has been observed in Hingham. 



The conglomerate of Hingham is composed for the most 



