Annual Meeting.] 
14 
[May 7, 
ham, Needham, etc., the successive flows of lava followed each other 
so rapidly as to form very thick sheets of volcanic rock without any 
intercalated sediments. Occasionally, also, the eruptions were ex- 
plosive and beds of volcanic tuff were formed, as at Nantasket, 
and in Brighton, Newton, Needham, etc. The igneous action was 
somewhat localized, continuing in some parts of the basin through 
the entire period of the formation of the conglomerate, while in 
other parts the lava is wanting or occurs only in the lower part of 
the conglomerate series. 
Alternations of the conglomerate with beds of both sandstone 
and slate are also of common occurrence, indicating oscillations of 
level during this period. But the downward movement prevailed, 
until finally the water became too deep and quiet and too remote 
from the shore, in this vicinity, to permit the formation of conglom- 
erate and sandstone ; but these coarse sediments were gradually 
replaced by slate during the dying out of the volcanic activity. 
These tranquil, deep-sea conditions must have continued for a very 
long time ; for the argillaceous sediments accumulate very slowly 
and yet the slate series has a thickness of from 500 feet to fully 
1000 feet or more. The slates are probably somewhat thicker than 
the conglomerates and were doubtless several if not many times 
longer in forming. Of the life existing in the sea at this time we, 
unfortunately, know but little, as the slate is very generally desti- 
tute of fossils ; and the full chronologic significance of the few or- 
ganic remains that have been obtained from the layers of limestone 
in the slate at Naliant is still undetermined. 
The deposition of the slate did not end in the same quiet way 
in which it began, by the gradual elevation of the sea-floor to form 
dry land. That would have involved a repetition of the shore con- 
ditions and deposits ; and the conglomerate series below the slates 
is certainly not repeated above. But during that long period of quiet 
deposition of the slate the subterranean forces were slowly gath- 
ering strength for renewed activity ; and when the weakened crust 
below the still unconsolidated sediments could no longer resist the 
growing horizontal thrust or pressure, it yielded ; and thus inau- 
gurated an important geological revolution. The slate and conglom- 
erate were powerfully compressed in a north and south direction 
and thrown into a series of gigantic folds, having a general east- 
west trend. Although they have suffered enormous erosion, these 
folds, when not drift covered, are still distinctly traceable, the an- 
