100 BULLETIX OF THE 



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 ridcres 

 from Branford northward to Meriden and beyond 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 modem 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 nictured 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 



