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silex and the overlying breccia. The contact surfaces are ex- 

 tremely irregular, and indicate much movement, if not fluency. 

 Small patches of conglomerate have also been observed repos- 

 ing upon the striped petrosilex here. These are quite readily 

 distinguished from the breccia, making it impossible to regard 

 the latter as the conglomerate, and thus remove this variety of 

 petrosilex from the Huronian formation, by referring it to the 

 Shawmut group. In some of these exposures the schistosity 

 approaches regular banding. 



On River Street, in Hyde Park, near the schistose petrosilex, 

 there is an outcrop of a stratified petrosilicious rock, which I intro- 

 duce here with considerable hesitation. There are two varieties, 

 greenish- white and dark-purple ; these are interstratified, but the 

 lines of bedding, which are quite regular and continuous, are 

 found chiefly in the former. Both varieties weather deeply, though 

 turning the knife on fresh surfaces. What appears to be the same 

 rock occurs on Blue Hill Avenue, south of the Neponset, where I 

 have represented it as covering a considerable area. The boun- 

 daries of this area are almost entirely conjectural ; and it is not 

 unlikely that true Huronian petrosilex is wholly wanting here. 

 The petrosilex east of Blue Hill Avenue, in Dorchester, is for 

 the most part, at least, very finely and evenly banded or strati- 

 fied. The bands do not exceed one-half a millimeter in thickness , 

 are quite continuous, and are all of a uniform whitish color, 

 the structure being visible only on the weathered surface. Lim- 

 ited portions of the rock are of a pinkish or even red color, and 

 more coarsely banded. The rock sometimes cleaves readily 

 in the direction of the bedding. The strike is N.E.-S.W. ; 

 dip, vertical. The petrosilex in Milton, between the Neponset 

 River and Pine Tree Brook, is very similar to that west of Back 

 Street. It is generally of a whitish color, although toward the 

 northern boundary there are irregular, ill-defined patches of 

 purple. The rock is nearly all distinctly banded, but the band- 

 ing is sometimes obscure or wanting in the purple patches. 

 The banding is mostly very even and continuous, consisting of 

 alternating grayish and purplish layers a millimeter thick, or 



