221 



The petrosilex is exposed only on the north-west side of Cen- 

 tral Avenue, forming the south-west end of a section which is 

 mainly conglomerate. The contact between the petrosilex and 

 conglomerate is straight and well defined. It strikes east- west, 

 and dips to the north 80° , being exactly parallel with the schis- 

 tosity or cleavage of the last-named rock ; while tliis imperfect 

 cleavage agrees perfectly in dip and strike with that observed 

 elsewhere among the slates and conglomerates of the Boston 

 basin. The contact just noticed almost certainly marks a line 

 of fault, and both it and the cleavage are entirely independent 

 of the bedding. The bedding, however, is much obscured by 

 the cleavage, though it can still be made out by careful observa- 

 tion. The strike and dip are the same as in the other masses 

 of conglomerate in the vicinity ; the strike being N.E.— S.W., or 

 nearly parallel with the avenue, and the dip very steep to the 

 south-east. The pinite in the conglomerate diminishes rapidly 

 as we recede from the outcrop of petrosilex. 



The slates in the railroad cut west of the Milton Lower 

 Mills Station can be traced east to where the railroad crosses the 

 river ; but the structure in this direction appears to be wholly 

 monoclinal ; and we find, between Adams Street and a point 

 one-fourth mile south of the railroad, several alternations of con- 

 glomerate, grits, rippled sandstone, and slate, all dipping 

 S. 20°-30° E. 50°-80°. The indications are that the syn- 

 clinal is here replaced by one or more strike faults, with the 

 downthrow on the north. 



Toward the north-east the rocks are concealed on the line of 

 this synclinal for a distance of two miles, appearing again in 

 North Quincy, nearly one-half mile north-east of the Old 

 Colony Railroad and one-fourth mile north-west of the road to 

 Squantum. Here the structure is once more clearly synclinal. 

 Advancing from the south, there are, first, several hundred 

 feet of conglomerate, with a high northerly dip, and becoming 

 smaller-grained in that direction ; and then, after a few feet 

 concealed, sandstone passing to true slate. There are at least 

 one hundred and fifty feet of the slate, traverse measure ; it is 



