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and the fifth conglomerate was formed. When the volcanic 
activity was again renewed over this area, the eruptions were, 
for the first time, partly of an explosive character, forming, as 
the Atlantic Hill section shows, beds of tuff alternating with 
beds or flows of compact and brecciated melaphyr. This marks 
the culmination of the volcanic energy, flow sueceeding flow, 
until they attained an aggregate thickness of several hundred, 
possibly five hundred, feet. This series of eruptions completes 
the conglomerate and melaphyr series as now developed in the 
Nantasket ledges ; but on the extremity of Rocky Neck we find 
evidence that the great melaphyr was finally covered by a 
sixth bed of conglomerate, and that by a flow of green, amyg- 
daloidal melaphyr resembling the second and third sheets of 
that rock. 
The volcanic energy finally died out ; and these alternating 
sheets of conglomerate, melaphyr, and porphyrite were probably 
covered by a great thickness of conglomerate and sandstone 
without interbedded lavas. But of this upper conglomerate 
series there are now no visible traces in the Nantasket Penin- 
sula, although Harding’s Ledge, as already explained, affords 
some evidence that it underlies the middle part of the beach, in 
the vicinity of Strawberry Hill. As the subsidence progressed 
and the water became deeper and the shore more remote, the 
deposition of the coarser fragmental rocks over this area 
gradually ceased, the conglomerate changing through sandstone 
to slate, which, we may fairly suppose, underlies almost the 
entire peninsula north of Atlantic Hill, and has a thickness of 
several hundred, possibly a thousand, feet, The deposition of 
the slate probably occupied a much longer time than that of 
the conglomerate series; but it was finally terminated by the 
period of disturbance during which the sediments of the Boston 
Dasin were strongly folded, faulted, and elevated to form dry 
land. It is to this geological revolution that we owe nearly all 
those structural complexities which make the stratigraphy of 
the Boston Basin such a difficult problem. This was also 
