Relations of the Igneous Rocks — Crosby. 39 
corresponding to the basic porphyry and the basic phase 
of the fine granite of the Blue Hills area. Third, the 
effusive acid lavas or felsites are, relatively, more abundant 
and far more varied in the Neponset valley than in the Blue 
hills. Fourth, the dikes of both acid and basic lavas so 
characteristic of the basal complex in the Neponset valley 
are practically or wholly wanting in the Blue hills. Fifth, 
the necks or actual vents of the effusive acid lavas are far 
more normally and typically developed in the Neponset 
valley than in the Blue hills, while the vents of the basic 
lavas are wholly wanting in the latter area. Sixth, the 
dikes of diabase, which are found in the eastern and north- 
ern parts, and are practically wanting in the main range of 
the Blue hills, are, in the Neponset valley, characteristic 
of all parts of the complex as well as of the overlying sedi- 
ments, no considerable area being free from them. Seventh, 
erosion has left in the Neponset Valley section of the com- 
plex, so far as it js now exposed, only very scanty traces of 
the original Cambrian cover. 
GENERAL HISTORY OF THE COMPLEX. 
After what precedes a brief statement will suffice here, 
the main purpose being a more systematic outline, pref- 
atory to the lithologic and structural details of the com- 
plex. As in the Blue Hills area, this area or part of the 
general batholite of eastern Massachusetts is believed to 
have been developed beneath a great thickness of Cambrian, 
and possibly of later, sediments, of which erosion has left 
only a few highly altered remnants. The thickness of the 
Cambrian cover was due primarily to extensive sedimenta- 
tion and secondarily and chiefly to severe or isoclinal plica- 
tion. The thickening of the super-crust thus determined 
was sufficient to induce a rise of the isogeotherms, an out- 
flow of the subterranean heat, so marked as to involve soft- 
ening and final fusion of the sub-crust or floor on which 
the Cambrian sediments were deposited, developing thus a 
great body of granitic magma, the corrosive action of which 
led to the absorption of considerable volumes of the sedi- 
mentary cover and gave rise, no doubt, to the normally 
* highly irregular and unconformable contact. 
This thickening of the super-crust and consequent great 
