Crosby.] 18 [January 16, 



this quartzite was buried before the conglomerate was made, and 

 none of the quartzite forming the floor of the basin can be 

 regarded as a possible source of the conglomerate pebbles. 



It is certainly probable that the slate and conglomerate were 

 deposited on a subsiding sea-floor. But, if so, then the forma- 

 tion of the conglomerate must have commenced first, although 

 the two rocks are in large part contemporaneous deposits, as 

 already explained. 



There is certainly no good reason to doubt that in its lithologi- 

 cal aspect the conglomerate is as old as the slate. Both rocks are 

 thoroughly consolidated, the cement of the conglomerate being 

 quite as strong as the slate and often approaching in hardness the 

 pebbles which it encloses. Both rocks are frequently locally 

 altered by intrusives. From the nature of the. case, the altera- 

 tion is most noticeable in the slate ; but it is sometimes very 

 marked in the conglomerate, as in that along Adam Street in 

 Quincy, which is altered by the Quincy granite. Even the cleav- 

 age structure which frequently appears in the slate, is matched in 

 the conglomerate, appearing in those varities in which pebbles of 

 pinite and slaty rocks predominate, the pebbles being flattened 

 by the pressure. The cleavage planes in the conglomerate agree 

 perfectly in dip and strike with those in the slate. 



I will now attempt to answer the particular objections which 

 Dr. Wadsworth has advanced against the foregoing views of the 

 stratigraphy of the Boston basin. Dr. Wadswoith's exceptions 

 to my conclusions are contained chiefly in two papers. 1 The first 

 paper relates to the rocks on and near Central Avenue, in Mil- 

 ton, in the area between Pine Tree Brook and the Neponset 

 River. The relations of the rocks in this interesting locality 

 were described by me in my " Contributions" (220 and 270), and 

 in the Amer. Jour. Sci., 1880 (3), xix, 116. 



I have stated that the oldest rock exposed on Central Avenue 

 is a ledge of purple f elsite which is altered superficially to green- 

 ish pinite. That after this alteration took place, conglomerate 

 was deposited over and around this ledge and that in the imme- 

 diate vicinity of the ledge it is largely made up of the debris of 

 both the unaltered felsite and the pinite. That the conglomer- 

 ate, which has been removed from the top of the felsite by ero- 

 1 Harvard University Bulletin, ii, 431, Proc. Boston Soc Nat. Hist., xxn, 130. 



