188 



for it is in the highest degree probable that these rocks are in 

 part contemporaneous deposits, slate in the deeper water and 

 conglomerate in the more shallow being formed simultaneously. 

 In other words, the deposition of the conglomerate began first, 

 but had not entirely ceased in some parts of the basin when the 

 deposition of the slate had begun in other parts ; so that 

 chronologically the two deposits overlap, and have not every- 

 where the same relative thickness. 



The principal facts proving that the conglomerate underlies 

 the slate are : (1.) At most points around the margin of the 

 basin the conglomerate comes between the slate and the crys- 

 talline rocks (classing the Shawmut group as crystallines), 

 precisely as the Shawmut group was shown to outcrop chiefly 

 along the line separating both the conglomerate and slate from 

 the more ancient formations, the true border of the basin. In 

 many places, it is true, this conglomerate border appears to be 

 wanting, even as represented on the map ; but the most im- 

 portant of these apparent exceptions occur where the rocks are 

 entirely concealed by drift, and we can not know with certainty 

 what they are. This is the case along nearly the whole of the 

 north side of the basin ; the nearest outcrops are slate, and 

 consequently this rock has been represented as extending to the 

 crystallines ; yet toward the south-west, in Needham, the con- 

 glomerate very plainly intervenes. In some of these excep- 

 tional cases, again, there are indications of faults, which, with 

 the downthrow on the side toward the slate, have carried the 

 conglomerate below the present surface. In still other in- 

 stances the contact of the slate with the bordering crystallines 

 may, with considerable probability, be regarded as true overlap 

 resulting from the progressive subsidence during and preceding 

 the deposition of the slate. This explanation seems applicable 

 at some points in the South Shore district. The geographic 

 relation here insisted upon is best shown around the three 

 peninsulas or tongues of the ancient formations (principally 

 Shawmut group) which project into the basin from the west. 

 The isolated masses, or islands, of the crystallines, too, where 



