Crosby.] 22 [January 16, 



The felsite ledge does not extend across Central Avenue ; but 

 Dr. Wadsworth says it would do so, " unless gratuitous hypoth- 

 eses are resorted to to support the felsite theory." If we cannot 

 legitimately suppose a ledge Fix feet high on one side of a street 

 without its counterpart on the other side, then the thorough- 

 fares of New England are lined with facts sustaining gratuitous 

 hypotheses. The felsite can be traced about one-fourth of the 

 way across the avenue, and I have no doubt that it underlies the 

 conglomerate on the opposite side. 



No structural feature of the conglomerate is plainer than its 

 cleavage. The rock is eminently shaly in planes parallel with 

 the fault ; and all the soft pebbles are flattened and nearly all the 

 hard pebbles are broken in directions requiring powerful com- 

 pression at right angles to the easy splitting or cleavage. It is 

 not easy to conceive a more perfect cleavage in a rock of this 

 character. As already indicated, the cleavage planes strike 

 about east-west and dip steeply to the north, agreeing in these 

 respects with the cleavage of the slate throughout the Boston 

 basin. One-half mile east of the conglomerate on Central Ave- 

 nue, on the east bank of the Neponset, are conglomerate and sand- 

 stone interstratified and dipping at a high angle to the south. 

 The sandstone is also traversed by very perfect cleavage-planes 

 dipping steeply to the north as usual. The conglomerate layers 

 being composed of hard pebbles, are insusceptible of cleavage ; but 

 if they were wanting, the cleavage of the sandstone might easily 

 be mistaken for stratification. Following the same belt of rocks 

 still farther east, we find on the shore of Squantum a slaty con- 

 glomerate with distinct cleavage which, as before, dips steeply to 

 the north, while the bedding is inclined to the south. 



Dr. Wadsworth has mistaken the cleavage of the Central Ave- 

 nue conglomerate for stratification; and he says " that pebbles 

 that would be readily affected by pressure lie across the supposed 

 cleavage planes exactly as they would if these were bedding 

 planes." I do not believe it is the general experience of geolo- 

 gists that pebbles in a conglomerate lie across the bedding planes. 

 Again, he says that the lines of coarser and finer sedimentation 

 prove that these planes of easy splitting are bedding planes. In 

 company with Prof. W. H. Niles, I searched carefully for these 



