62 
face now to the south and then to the north, evidently signify 
nothing but unequal erosion, appearing always on the side of 
the harder and more resistant rock, which is usually melaphyr. 
Faets will, however, be aecumulated in the following pages 
pointing to the correlation of the conglomerate of Conglomerate 
Plateau with a horizon below that of the conglomerate on the 
extremity of Long Beach Rock; and hence to the conclusion 
that the downthrow is not only on the north, but that it must 
exceed the entire thickness of the melaphyr series of Atlantic 
and Centre Hills, — four hundred if not five hundred feet. 
Although melaphyr occurs abundantly in the central as well 
as the coastal area, it is of a different and distinctly more basic 
type. It is very generally and often profusely amygdaloidal, 
rarely distinctly brecciated and never, so far as observed, ac- 
companied by beds of tuff. The abundance of conglomerate 
south of the great fault, as well as the numerous faults, still 
further contrasts the two areas. It is, of course, quite possible, 
if not probable, that some of the faults of the coastal area have 
escaped discovery on account of the uniform character of the 
melaphyr in Atlantic and Centre Hills ; and the great apparent 
thickness of this melaphyr ought, perhaps, to be regarded as 
largely due to repetition by faulting. 
'The alternations of the conglomerate and melaphyr in the 
central area are so frequent that it will be more convenient to 
describe these rocks together, following the order of the ledges 
or topographic features. 
East shore of Nantasket Bay, Crescent Hill and Marsh 
Island. — These localities, forming a narrow belt across the 
western end of the central area, between the railroad and Mela- 
phyr Plateau, may be conveniently described together, since, as 
the map shows, they present, as the result of parallel faulting, 
simply repetitions of certain beds of conglomerate and melaphyr. 
This tract is bordered by two well-marked north-south faults, 
and this primary fault-block is divided by no fewer than twelve 
transverse fractures, with the slips alternating in direction, and 
indicating the unequal rising and sinking of V-shaped blocks 
