29 
a thick sheet of sedentary soil or chemical detritus, it is obvious 
that the more basic rocks, like diorite and diabase, must have 
been more deeply and thoroughly deeayed than the more acid 
rocks, like granite and felsite, the latter being intermediate in 
this respect between the former and the purely silicious rocks— 
the quartzites. The débris of the diorite is, therefore, to be 
sought in the slate rather than in the conglomerate ; and we 
must recognize the principle that the occurrence of older rocks 
in the conglomerate is not in the order of their abundance во 
much as of their chemical stability. 
Limited layers of finer sediment—grit and sandstone—by 
reference to which the dip and strike may be determined, occur 
rather sparingly in most of the Nantasket ledges, as in Long 
Beach Rock, Green Hill Ledge and Great Hill; although they 
are practically wanting in several of the largest masses, includ- 
ing Conglomerate Plateau and Conglomerate Hill. The con- 
glomerate is throughout very firm and thoroughly consolidated, 
breaking, usually, without reference to the contours of the 
almost adamantine pebbles. In fact, the paste, including the 
intercalated sandstone layers, is often so intensely hard as to 
suggest that the volcanic heat of the interbedded lavas has been 
an important agent in lithifying these ancient gravels. This is 
undoubtedly a true cause, but still it does not afford an entirely 
adequate explanation of the phenomena, since the conglomerate 
beds are usually well indurated through their entire thickness, 
even the lower layers of gravel, which were deposited upon a 
cold and eroded surface of lava, being sensibly as hard in most 
cases as the upper layers, which were covered by a stream of 
incandescent lava. There are, on the other hand, many facts 
Which indicate that the intense lithifieation is largely due to the 
deposition of secondary silica over and between the grains com- 
posing the paste, thus virtually changing the arenaceous matrix 
of the pebbles to quartzite. This interstitial silica is especially 
obvious where it takes the form of distinct segregations of' red 
Jasper, as in the western part of Nantasket and on Rocky Neck. 
