Crosby.] 20 [January 16, 



neighborhood of faults. Several good examples of crush breccia 

 are to be seen in the slate on the railroad, near Central Avenue ; 

 and in this way limestone and many eruptive and crystalline rocks 

 may take on a conglomerate structure. The undoubted felsites 

 of eastern Massachusetts very commonly contain pebbles ; and a 

 breccia texture due to included fragments is one of their most 

 common characteristics, as Dr. Wadsworth will probably agree. 

 Possibly some of the supposed pebbles in the Central Avenue 

 felsite may be accounted for in this way. This rock certainly 

 looks like a felsite, and I think we must regard the conglomerate 

 theory of its origin as uncalled for until it is proved to contain 

 pebbles which cannot be explained in either of these ways. Dr. 

 Wadsworth says the microscopic evidence bears out his view 

 that the felsite is an altered argillaceous conglomerate. My ex- 

 aminations of the thin sections, on the contrary, show that it is 

 essentially similar to other felsites of this region. Most litholo- 

 gists will admit that there are rocks whose origins cannot be 

 determined by the microscope ; and I believe that this highly 

 weathered, crushed felsite belongs to that class ; and I also believe 

 that it presents no characteristics, either microscopic or macro- 

 scopic, which are not exhibited in other and undoubted felsites 

 of this region. In this opinion I am sustained by Dr. N. F. Mer- 

 rill, who kindly examined the thin sections and hand specimens 

 of felsite from this and the adjoining ledges without knowing 

 where they came from or that they were the subject of any con- 

 troversy. 



About three-fourths of a mile from this locality, on the north 

 side of the river and near Mattapan, the contact of the conglom- 

 erate and felsite is exposed again and very satisfactorily. The 

 felsite is distinctly banded and the banding is perpendicular to 

 the contact, which is vertical and apparently due to faulting. 

 The conglomerate is a distinct breccia chiefly composed of frag- 

 ments of this same felsite and of amygdaloid, which occurs near 

 by. Now I think the burden of proof rests upon those who 

 claim that the phenomena presented at this point are essentially 

 different from those on Central Avenue. 



If the Central Avenue felsite is really an altered conglomerate, 

 then the soft pinite masses must be the least altered portions, 

 since they are most like the unaltered conglomerate, but they 



