— EE ERE - 
Or 
25 
a) ә, . . 
The assigned thicknesses are in part actual measurements ; 
but chiefly estimates and approximations. They are intended, 
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| also, to give some idea of the variations in thickness observed in 
| tracing the individual beds from one part of the field to another. 
| While it must be admitted that the data upon which this 
| table rests are in some respeets much less conclusive and more 
| ambiguous than could be desired, it may be fairly considered | 
a conservative determination as regards the number of distinct 
terranes involved; since in all cases where the indications 
appeared to be evenly balanced that interpretation of the facts 
has been preferred which avoided multiplying the alternations 
of conglomerate and lava. It is quite possible, indeed, that in 
some instances what have been regarded as parts of one continu- 
ous sheet of lava are really entirely distinct and, perhaps, not | 
even synchronous flows. This view would, however, seem to | 
L carry the further supposition that some of the flows failed to | 
extend over the entire field. This is not, « priori, improbable ; 
| though few facts have been observed which seemed to demand 
this explanation. Some of the tuff beds are unquestionably 
very local, the underlying and overlying melaphyrs coming 
P together abruptly at their margins; but, although carefully | 
sought, no corresponding indications have anywhere been | 
detected in the melaphyr, except in the case of the flow on | 
Atlantic and Centre Hills which appears to be terminated on | 
the west by the junction of the underlying and overlying tufts. 
It is very clear, however, that, as already stated, the melaphyr 
lowed each other in quick succession over the same area. This 
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flows are composite in some cases, several eruptions having fol- | 
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is conspicuously true of the fourth melaphyr, the great. sheet | 
d | 
forming Atlantic and Centre Hills, ete. 
Although, as the map shows, few of the Nantasket faults | 
san be classified as distinctly longitudinal or transverse, the | 
d й | 
greater number are either approximately east-west or north- | 
H . Hy 
south in direction. It is also noticeable that the blocks bounded 
by the principal faults are in several instances broken into | | 
quite narrow wedges by a series of parallel east-west faults, 
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Hl 
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