68 
eighteen feet. The lower melaphyr very clearly rises to the 
south or toward the fault, and again to the north, giving the 
syncline already described. The fault trends N. 75° E., being 
in line with dike 37 in Great Hill; and it can be readily traced 
directly across the entire breadth of the hill, following approxi- 
mately the boundary line between the conglomerate and upper 
melaphyr, and breaking this contact almost as distinctly on the 
eastern as on the western face of the hill. About midway be- 
tween this fault and the southern end of the conglomerate is a fifth 
fault, which is the counterpart of the last, the downthrow being to 
the north and about 15 feet. It plainly jogs the contact between 
the conglomerate and upper melaphyr; and trends N. 75? W., 
being in line with dike 44 on Great Hill. Finally, at the ex- 
treme southern end of the conglomerate there appears to be à 
sixth fault, probably parallel with the last, but with the down- 
throw to the south. The amount of the throw, if to the south, 
is certainly 10 if not 15 feet, being enough to throw the con- 
glomerate entirely out of sight. If the throw were to the north 
it would need to be 25, and more probably 50, feet to do this. 
'The fact that the melaphyr under Crescent Hill, although 
rising to the west, can not be found, not even the slightest trace 
of it, on Great Hill, is alone sufficient to prove an important 
north-south fault between the two hills, with the downthrow to 
the east ; and the further faet that not one of the dikes in Great 
Hill ean be traced in Crescent Hill points to the same conclu- 
sion. Indeed, the contrast presented by the opposite walls of 
this narrow defile is one of the especially surprising and puz- 
zling features of Nantasket geology. The very abrupt way in 
which the conglomerate of Crescent Hill terminates on the east 
proves that the hill is boundéd in this direction also by an im- 
portant north-south fault, the downthrow probably being as 
before on the side of Crescent Hill, or to the west. 
Marsh Island is a large ledge of conglomerate and melaphyr 
completely isolated by the eastern marsh. Geologically it is 
but a repetition, at a lower level and without important, dis- 
placements, of Crescent Hill. The conglomerate, which forms 
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