187 
probably a period of intense igneous activity, the numerous 
dikes of diabase traversing the ledges of Nantasket and other 
parts of the Boston Basin appearing to date from this time. 
The dikes were probably, in many cases, the feeders or channels 
of supply of effusive eruptions ; but these surface lavas of every 
form, not being protected by later sediments, have, at least in 
in the Nantasket district, been long since completely swept 
away by the agents of erosion, which during all subsequent 
time have worked unceasingly and so efficiently that not only 
have these ancient volcanoes been destroyed but the very 
foundations on which they stood. In southern Nantasket the 
entire thickness of the slate series and probably, on the average, 
half of the conglomerate series have been removed; and over 
broad areas erosion has exposed the original granite floor or cut 
deeply into it. 
AGE OF THE NANTASKET ROCKS. 
Up to the present time no fossils have been found in the Nan- 
tasket ledges. It is, of course, improbable that any ever will be 
found in the conglomerate and volcanic series; but we may 
reasonably entertain the hope that the slate ledges will yet afford 
us some clue to their geological age. Fragments of а some- 
what calcareous slate have been observed in the drift of Straw- 
berry Hill, and suggest the possibility that beds of impure 
limestone (of organic origin, if not actually fossiliferous) underlie 
the northern end of the peninsula. It seems impossible to feel 
quite sure that the hard massive slate on the railroad, northwest 
of Rockland Hill, whieh has been referred provisionally to the 
bed of tuff outcropping at the base of Atlantic Hill, is not really 
an isolated exposure of the Cambrian slate of Braintree and 
Weymouth. The Weymouth beds are now referred by 
Walcott, provisionally at least, to the Lower Cambrian ; and 
the Braintree beds to the Middle Cambrian!. The granite and 
diorite are undoubtedly here, as elsewhere in the Boston 
! Tenth Ann, Report U.S. Geol. Survey, p. 567. 
