189 



not clearly eruptive, are usually entirely surrounded by a zone 

 of conglomerate. 



(2.) The conglomerate almost invariably dips towards the 

 outcrops of the slate, and in not a few cases can actually be 

 seen to pass beneath them. The reverse of this sometimes 

 occurs, as in the Beacon Street section near Newton Centre 

 (PL 5, fig. 1) ; but I shall endeavor to show on a subsequent page 

 that this may be properly explained as due to an inversion of 

 the beds (see also PI. 5, fig. 3), — an explanation which, how- 

 ever, will not apply in the far more numerous cases where 

 the conglomerate is the underlying rock. (3.) In fact, save 

 where occurring on the margins of the basin or exposed through 

 the agency of faults, the conglomerate never comes to the sur- 

 face except through denuded anticlinals from which the slate 

 has been eroded away ; though in some cases the conglomerate 

 too has been worn through, bringing into view the crystalline 

 axis beneath. 



The statement has been made that the slate as a whole is 

 characterised by steeper dips than the conglomerate, implying 

 greater age ; but I have not been able to verify this observation. 

 Both these rocks are for the most part highly inclined, often 

 vertical, and yet they generally appear to be entirely con- 

 formable. The large mass of conglomerate in Dorchester, 

 Roxbury, West Roxbury, and Brookline, usually exhibits very 

 gentle dips, and in some parts is quite horizontal ; but this is 

 accounted for by the fact that it here covers the crest of a very 

 broad anticlinal ; besides, this area of slightly inclined con- 

 glomerate is offset by the gently dipping slate of Somerville 

 and Cambridge. I have prepared lists of all the dips of both 

 elate and conglomerate recorded in my notes, but fail to dis- 

 cover any decisive difference between the averages. 



The supposed occurrence of pebbles of the slate in the con- 

 glomerate has been confidently appealed to as conclusively prov- 

 ing the greater antiquity of a portion at least of the former rock ; 

 but after a critical examination of many hundreds of these ' ' slate 

 pebbles " I am fully convinced that I have not seen a single 



