45 
the preceding sections are based; not forgetting that the 
highest interest attaches to the relations and especially to the 
contact phenomena of the bedded rocks—conglomerates and 
lavas. For present convenience, as well as in the interest of 
future students of the Nantasket ledges, each principal mass or 
area will, as regards the sedimentary rocks and associated lavas, 
be described somewhat independently and in topographic order, 
reserving the general correlation, the complete elucidation of 
the stratigraphy, until the close of this systematic itinerary. 
The structural details of the dikes, or the intrusive igneous rocks, 
will then be presented, in both chronologie and topographic order. 
The Atlantic Shore or Coastal Area. 
This area, extending from Nantasket Beach to Black Rock, 
includes the most accessible and the most frequently visited of 
the Nantasket ledges ; and it is а fortunate cireumstance that, 
although the melaphyr largely predominates here, some of the 
more characteristic features of Nantasket geology аге well ex- 
hibited at this natural starting point. The granite, however, 
is Wholly wanting; and its interesting relations to the con- 
glomerate cannot be observed in this part of the field. 
part of Long Beach Rock is a somewhat isolated, half-tide 
ledge of conglomerate. It contains many pebbles, mostly 
small, of granite and felsite, and also many larger pebbles of 
different. varieties of melaphyr. From the latter we may infer 
that this bed is underlain by melaphyr, and that it is probably 
not the lowest or basal conglomerate. Eight or ten feet below 
the top of the conglomerate is an intercalated layer of fine, 
hard, red sandstone, eighteen inches thick; and conformably 
overlying the conglomerate, are six to twelve inches of a beau- 
tifully banded, greenish slate of flinty hardness. The sandstone 
and slate show that the strike is N.65° E.*, and the dip S.E. 
15990), 
1 P Н : : H . 1 soe 
All azimuth directions given in this paper are referred to the true meridian, 
