1884.] 27 [Crosby. 



Here we have a whole series of facts demanding a synclinal 

 position for the slate ; and what are the facts opposed to this 

 view? Simply this: the northern conglomerate rests uncon- 

 formably upon the slate at Newton Centre. But the same con- 

 tact is perfectly conformable at the Chestnut Hill Resevoir. 

 Besides, every geologist knows that unconformable contacts may 

 be due to disturbance and faulting as well as to erosion ; and I 

 think Dr. Wadsworth has failed to prove that the Newton Centre 

 contact necessarily belongs to the latter class. He says this is 

 not a fault, because the slate and conglomerate are united in a 

 solid mass. But this certainly is no unusual feature of faults. 

 Why should not two rock surfaces that have been in contact 

 under enormous pressure during long ages be firmly united? 

 I find, however, on a careful reexamination of the contact, that 

 at most points the slate and conglomerate are not united. 

 On the other hand, if, ns so many important considerations re- 

 quire us to suppose, this is an overturned synclinal, then the pli- 

 cating force must have been greatest irom the north ; i. c, in the 

 right direction to produce jnst such oblique fractures as the one 

 in question. And it is important to remember that a slip of a 

 few feet or yards along this irregular fracture would give rise to 

 all the unconform ability observed. 



That oblique faulting has occurred along this boundary line 

 between the slate and conglomerate is proved beyond the possi- 

 bility of a doubt by the fault exhibited in the east bank of the 

 Chestnut Hill Reservoir, and which is described and figured in 

 my " Contributions." Now all that the synclinal theory requires 

 is that the Newton Centre unconformability should be explained 

 in the same way as that at the reservoir on the same strati- 

 graphic line. If, as Dr. Wadsworth supposes, this northern belt 

 of conglomerate is newer than the slate, it certainly is a strange 

 circumstance that it shows much steeper dips and greater distur- 

 bance than the slate on which it rests. 



Dr. Wadsworth agrees with me that the Newton Centre slate 

 is probably of the same age as the Paradoxides slate in Braintree. 

 But the Newton Centre slate is unquestionably underlaid by the 

 broad belt of conglomerate on the south of it ; and hence Dr. 

 Wadsworth virtually admits that there is in the Boston basin a 

 great formation of conglomerate older than the Primordial slates. 



