Crosby.] 24 [January 16, 



admitted facts that the granite is eruptive at some points through 

 the slate, and that granite pebbles are abundant in the conglom- 

 erate, asks how it is possible, in view of these facts, to regard the 

 conglomerate as underlying the slate? In reply I would say that 

 the granites surrounding the Boston basin have an area of nearly 

 200 square miles and present many varieties ; that it is in the 

 highest degree improbable that they are all of the same age ; 

 and that at some points, as in Hingham, the granite is undoubtedly 

 eruptive through both the conglomerate and slate, the same 

 conglomerate holding pebbles of granite. Pebbles of basic erup- 

 tive rocks — trap, melaphyr, amygdaloid, etc. — are common in 

 the conglomerate ; but we cannot argue from this fact that the 

 conglomerate is newer than all the rocks of this class, because the 

 conglomerate is traversed in all directions by dikes and eruptive 

 masses of these same exotic rocks. 



The second paper by Dr. Wadsvvorth relates to the unconform- 

 able contact of the slate and conglomerate on the north side of 

 Beacon Street, Newton Centre. It does not appear from this 

 paper that the unconformability of the slate and conglomerate at 

 this point had been previously observed, but Dr. Wadsvvorth 

 describes it as if it were an original discovery and distinctly says 

 that " the proof of the unconformable superposition of the con- 

 glomerate upon the argillite has been overlooked by Mr. Crosby." 

 Yet, in my Contributions this unconformability is described in 

 greater detail than by Dr. Wadsworth and on several different 

 pages I have stated that the contact is clearly unconformable. 

 What I have denied, and still deny, is that the unconformability 

 is due to erosion and that the slate is older than the conglomer- 

 ate. 



It is a common saying that by isolating facts, or taking them 

 out of their normal relations, we can prove almost anything. 

 This adage is especially true when applied to the geology of 

 such a complicated district as the Boston basin. Taken by 

 itself, the ledge on Beacon Street does show all that Dr. Wads- 

 worth claims, except that it will require stronger evidence than 

 has yet been offered to prove that the fragments in the conglom- 

 erate supposed to be pebbles derived from the underlying slate 

 really are such. 



There is little room to doubt that the slate at Newton Centre 



