Lane.] 
94 
[Dec. 5, 
from Nahant : prehnite, chlorite, dolomite, calcite, epidote, quartz 
and lignified asbestus (metaxite, found near Maolis), prase, chert 
(probably an erroneous determination of the garnet bands), dato- 
lite (which I have not found) and corundum (which must have been 
from an erratic). 
The main structural feature is the synclinal indicated by the sed- 
iments, which dip 47° N.W. (str. 40° N.E.) at Nahant Head, and 
at Little Nahant dip 70° S.E. (str. 52° N. E.) . 
Signs of this basin structure are also seen in the great diabase 
sheets, and to a certain extent in the petrographic varieties, but 
most especially in the joints and dikes. 
One set of joint planes are always far more nearly perpendicu- 
lar to all the rest than any other. They have generally less dip 
than the others, though near the contact with the sediments they 
are parallel to it, and they are seldom or never used by dikes, even 
where, as at Little Nahant, they stand at a high angle. They are 
the original horizontal joints of the sheets, of diabase allowing 
them to fold as do bedding planes— -only with difficulty, as sharp 
flexures, crush-lines and numerous small faults show. The aver- 
age strike and dip of dikes show the effect of folding plainly. 
The faults are mainly parallel to the axis of the synclinal and up 
the hade, as the way they throw the dikes shows (the two parts 
being moved each toward the acute angle of intersection). A few, 
of generally greater throw, are in other directions, e. g., one 450 
feet east of Pulpit Rock. Most of the faulting seems to have 
taken place since the intrusion of the dikes, as the youngest set 
(strike N. 7° W.) is much affected thereby. 
The separation of the dikes into groups is difficult, as dikes of 
like petrographic character have come in at different times, and the 
same set of joint planes has been repeatedly used. Good illus- 
trations of complication are seen along Canoe Beach. However, 
the N. 7° W. joints seem to have been the last used, being injected 
mainly with very fine grained trap. Next older, are dikes along 
the N. 80° W. joints, not generally very basic. Still older are 
dikes whose strikes bisect the angle between the strikes of the 
first two sets, and a set, consequently, about parallel and perpen- 
dicular to the synclinal axis. This in connection with Daubree’s 1 
experiments lead us to look on the younger dikes as filling joints 
1 Dana’s Geology, 3d ed., p. 802. 
