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excellent training ground for the Harvard Summer School of Geology. 
There is, however, still difference of opinion as to which of the trap 
sheets are of extrusive origin, and it has therefore seemed advisable 
to examine all the evidence thus far collected which bears on this 
question. ; 
2.—Means of Distinguishing Intrusions and Extrusions. 
Our belief is that the eastern traps are extrusive sheets, which were 
poured over the floor of the Triassic estuary from various and undis- 
covered vents at several times during the deposition of the bedded 
members of the formation; that three of the sheets attained areas of 
many square miles, — perhaps of several hundred square miles, — the 
second of the three being the sheet now seen in the main line of ridges 
from Branford northward to Meriden and beybnd to the Massachusetts 
line, while the first and third constitute the anterior and posterior ridges 
respectively. It is probable, also, that certain other eruptions occurred 
later, although the outcrops of their flows are not yet well correlated. 
If such be the facts, we should expect from our knowledge of existing 
lavas to find many indications of the contemporaneous origin of these 
sheets. Deposits of ashes and bombs may reveal the locus of eruption. 
More or less disturbance may have been created in the unconsolidated 
sediments as the lava flood advanced over them at the bottom of the 
estuary. Successive flows or intermittent advances of a single flow may 
have quickly followed one another, forming a composite sheet of lava. 
While the middle part of a flow would be relatively dense, the upper 
part would be vesicular, after the fashion of modern flows, and the sur- 
face might exhibit the ropy or clinkery character of lava streams. After 
the eruption, the igneous sheet would be gradually buried by the con- 
tinued deposit of sediments that settled slowly down in all the cavities 
and inequalities of the surface, thereby acquiring a stratification in mi- 
nute accord with all its irregularities. Where the waves and currents of 
the ancient estuary were strong enough, clinkery fragments may have 
been moved about on the surface of the sheet from the more exposed 
situations, and carried to the deeper, quieter water, there settling down 
with finer detritus from a more distant source. 
On the other hand, if the lava sheets that we have pictured as extru- 
sive were in reality intrusive, nearly every feature would be changed. 
The contrasted features of the two kinds of sheets must surely be 
distinct enough for preservation and detection. We have therefore 
