MUSEUM OF COMPARATIVE ZOOLOGY. 297 



The force which caused the intrusion of these sheets can hardly have 

 been the expansion of vapors and gases, which plays so important a 

 part in modern volcanoes ; for evidence of such expansion (vesicular 

 structure) is wanting. It was therefore more likely mechanical, and 

 connected as already suggested with the downfolding of the troughs 

 rather than with the subsequent elevation and tilting of the sandstones. 

 So far as this holds good, it also points to a Triassic date for the intru- 

 sions as well as for the ovei'flows. 



Overflow Trap Sheets. — By far the greater number of trap sheets seem 

 to be of overflow origin. The proof of this origin is more or less com- 

 pletely established for the following examples. 



The high trap cliffs of the Bay of Fundy are described by Dawson as 

 overflows, but he gives no account of their upper contacts, and his sec- 

 tion (here copied, fig. 18) shows much more irregularity than any that 

 I have found in Massachusetts and farther south. But amygdaloids 

 are very common in Nova Scotia, and these seem limited to overflow 

 sheets. On Grand Manau the trap and amygdaloidal beds conform to 

 the adjoining sandstones (Bailey and Matthew, Verrill). The Connecti- 

 cut valley gives many examples. Farthest north is Deerfield Moun- 

 tain, as recently described by Emerson and as represented in this 

 paper (A) ; the long range beginning at Belchertown, Mass., including 

 Mounts Tom (B) and Holyoke, and extending to the Hanging Hills (E) 

 by Meriden, Conn., has been fully shown to be an overflow in its north- 

 ern part, and it can hardly be of other origin farther south. Its lateral 

 ridges are probably all overflows as well. Hitchcock's observations 

 applied to the posterior ridge on the back of Mount Tom, and should 

 have left no doubt of its mode of formation. 



In Connecticut, much observation is still necessary to decide finally 

 on the origin of the numerous ridges. The evidence that favors the 

 intrusive origin of all Percival's western line of elevation has already 

 been stated. Equally good evidence may be found to show that all the 

 large and most of the small sheets of the eastern lines of elevation are 

 overflows. The frequent occurrence of indurated sandstones, and the 

 general absence of amygdaloids, in the west, contrast strongly with the 

 lack of evidence of distinct metamorphism, and the frequent mention of 

 amygdaloids, and trap and amygdaloid conglomerates, in the east. Toket 

 Mountain (E. II.) is described as dense trap at the base, amygdaloidal 

 at the top, and overlaid by friable red shale (Percival, 338). Lamenta- 

 tion Mountain (E. III. 5) has swells of amygdaloid on the back, overlaid 

 by shale (352). The anterior range is separated from the main range 



