15 
[Annual Meeting. 
ticlines being marked usually by belts of conglomerate and the 
synclines by belts of slate. The great conglomerate anticline run- 
ning through the middle of the basin is the central arch in a some- 
what symmetrical series of folds. The Dorchester and West Rox- 
bury syncline on the south matches the Boston, Brookline and 
Newton syncline on the north ; and the complex and broken an- 
ticline of the Neponset Valley matches the similar Brighton and 
Newton anticline ; while the folds believed to exist in the drift-cov- 
ered area of slate between the Neponsite anticline and the Blue 
Hills correspond to those in the similar area of slate between the 
Brighton and Newton anticline and the highlands north of the basin. 
The strata were extensively broken and faulted, as well as folded ; 
and in some parts of the basin the displacements are a more im- 
portant structural feature than the plications. The sharply defined 
northern and southern margins of the basin, as previously stated, 
are best explained as due to profound faults along these lines. The 
downthrow in each case was on the side toward the center of the 
basin, and these two great displacements must be. regarded as of 
primary importance in the geological structure of this region ; for, 
virtually, they present on the upthrow side two solid walls of crys- 
talline rocks between which the great central area of sedimentary 
and volcanic rocks has first settled down and then suffered com- 
pression, as in a vise, by the approach of these north and south 
walls, producing the great folds already noticed and most of the 
minor faults of this region. This was also an epoch of great igneous 
activity, many of the fault and joint fissures being injected by high- 
ly basic liquid rock (diabase), forming the great network of dikes 
traversing not only the sedimentary, but also the crystalline rocks. 
In the slate, especially, the trap has often followed the bedding 
planes, producing intrusive sheets, sometimes of great magnitude, 
as at Nahant and the outer islands. If any surface flows or volcan- 
ic cones attended the formation of the dikes, they have been com- 
pletely effaced by subsequent erosion, leaving no sign of their 
former existence. Perhaps the most important result of this geo- 
logical revolution was the elevation of this region ; and so far as 
we have any evidence it indicates that the Boston Basin has been a 
land area, an erea of erosion, during all subsequent geological time. 
The sedimentary rocks have thus been greatly reduced in both 
volume and area. That they formerly extended far beyond the 
present sharply defined limits of the basin there can be no doubt, 
