174 



in the greenish, slaty matrix. The relations of these rocks to the 

 conglomerate are often very puzzling. The amygdaloid pebbles 

 in the conglomerate, however, make it impossible that any doubt 

 should arise as to the true sequence of the two formations. 



Distinct stratification is rarely met with in the amygdaloid of 

 this district. And yet the very evident slaty structure observ- 

 able at most points ; the parallelism of the areas with the strike 

 of the conglomerate, a feature often noticeable, too, in the dis- 

 position of the individual outcrops ; the general uniformity in 

 the lay of many of ^the ledges, on both a large and small scale, 

 indicating a nearly constant dip ; and the paucity of indubitable 

 evidence of extravasation, convince me that, considered as a 

 whole, this must be regarded as essentially a stratified rock. 



Shawmut Group in Dedham, Hyde Parle, Dorchester, 

 and Milton, — The breccia in West Dedham has been de- 

 scribed, ante, pp. 80-81. The rocks of this age in the other 

 three towns named above may be conveniently and properly re- 

 garded as forming one large irregular area, since, as a matter 

 of fact, they would do so but for the overlying conglomerate. 

 Both divisions of the Shawmut group are found in this area, 

 and they are of approximately equal extent. The amygdaloid 

 occurs mainly on the east side, and the breccia on the west 

 side, of the New York and. New England R.R. ; and there 

 are indications along the line of this railroad of a lithologic 

 passage from the one division to the other. The breccia here 

 is very variable in texture and color, and is often distinguish- 

 able with difficulty, if at all, from the petrosilex, especially if 

 the latter rock is of either the schistose or brecciated varieties, 

 for the schistose structure cannot be regarded as wholly foreign 

 to the breccia. (See under petrosilex.) To the westward of 

 the Boston and Providence R.R., and south of West Street, it 

 is doubtless more abundant than represented on the map. 

 East of this railroad, however, I have marked only a few of 

 the many islands of petrosilex occurring in the breccia, — knobs 

 and bosses of the underlying, but evidently not deeply buried, 

 formation, which erosion has exposed to observation. 



