197 



clastic rock. The fact that the iVagiiicnts or so-called pebbles 

 show a gradation indistinctness from those that arc very sharply 

 defined to those that are perfectly l)len(led Avith the en(;losing 

 felsite, is only what would he expected when fragnu>nts of 

 glass (obsidian) are enveloped in melted glass. The only 

 particularly obvious indications of chemical change in the felsite 

 since its eruj)tion are, first, the red color, which may be 

 original, but is probably due in part at least to the peroxidation 

 of iron during devitrification : and, secondly, the occurrence in 

 the rock of inconspicuous streaks and masses of ([uartz, either 

 vitreous, chalcedonic, or jaspery, — silica replacing the less 

 stable portions of the glass. Red felsites are not unconnnon in 

 eastern Massachusetts ; but the only occurrence at all closely 

 resembling the Hingham felsite is the red felsite near the 

 Neponset River, in Ilydc Park. Structurally they are strik- 

 ingly similar, except that the banding of the Hyde Park variety 

 is rather coarser, and it is more generally brecciated ; but the 

 Hingham felsite, with its deeper and brighter color is the more 

 beautiful of the two. 



THE BEDDED ROCKS OF NORTHERN HINGHAM. 



GENERAL RELATIONS AND ORIGIN. 



It is unnecessary to repeat what has been stated under this 

 heading for the Nantasket area ; but the one to[)ic may be 

 regarded as supplementary to the other. The sedimentary 

 rocks of Hingham present, as previously stated, a greater variety 

 than those of Nantasket, embracing besides the conglomerate 

 many intercalated beds of sandstone and slate of both brownish 

 and greenish tints, and the great body of gray slate overlying 

 this conglomerate series. The cont(nnp()raneous volcanic rocks 

 or effusive lavas, on the other hand, are less varied, including 

 no acid or fragmental varieties, but consisting wholly of rather 



