92 



the Blue Hill petrosilex and the small area of this rock in 

 Hingham, which is the last that remains to be described. The 

 petrosilex in Hingham is on the west side of Hingham Harbor, 

 and is found chiefly in the form of loose masses in the meadows 

 adjacent to Lincoln Street, east of Thaxter Street. Presi- 

 dent Hitchcock described the rock as occurring here in situ ; 

 but for many years no true ledge was visible ; recently, how- 

 ever, Mr. T. T. Bouv6 has re-discovered the original ex- 

 posure under the road-bed of Lincoln Street at about the point 

 where this street approaches nearest to tide water. The ledge 

 had been blasted away in grading the street, and thus rendered 

 inconspicuous. This petrosilex has a distinctly schistose 

 structure ; yet it is not coarsely schistose, as in Hyde Park 

 and other places, but very finely, like the petrosilex at 

 Cliftondale in Saugus, and on Oliver Street in East Maiden. 

 Portions of it, however, are less brecciated and more vividly 

 colored than in either of these cases. It is the handsomest 

 rock which I have met in Eastern Massachusetts. The color 

 is deep red, brighter than in the Hyde Park rock, and matched 

 only in some portions of the " Saugus jasper." The schistosity 

 is very fine, with now and then a coarser layer ; these coarser 

 layers, especially, are very even and continuous, and appear 

 to link this structure with the regular banding. Portions of 

 the rock hold petrosilex pebbles, and the attendant phenomena 

 are apparently the same as in Hyde Park. 



Petrosilex is not known to occur in situ to the southward of 

 this Hingham locality ; granite is, apparently, the all-prevailing 

 rock in that direction, and I have mapped it as extending with- 

 out interruption to the border of the formation. That region, 

 however, is so extensively drift-covered as to render this purely 

 negative evidence of the absence of petrosilex of little value ; 

 wliile some observations made by Prof. Hitchcock incline me to 

 the opinion that petrosilex probably occurs in situ within the 

 limits of Plymouth, and possibly of Carver and Kingston. In 

 the State Cabinet collected by Prof. Hitchcock are specimens 

 of petrosilex from Manomet Hill in Plymouth, and in his 



