SUMMARY OF HISTORY OF THE TRIASSIC BEDS. 499 



The beds containiug reptile tracks are almost without exception above 

 the great trap sheets, and in most cases not very far above them vertically. 

 Some of the localities situated far to the east have been brought up by 

 faulting. These central ex})0.sed mud flats seem to have been caused by 

 shallowing of the waters, which resulted from the flowing of the great sheet 

 out over the bottom. 



The present dips are the result of three actions difficult to separate: 



(1) Deposition upon an inclined plane, especially that between the 

 central shallower portion and the deeper portion on the border. This 

 seems to be the case across Hatfield and Deerfield, on the western side 

 of the basin, where the finer central beds dip slightly west toward the 

 coarser beds near the shore, and across South Hadley, Springfield, and 

 farther south, where the finer central beds have a low dip eastward toward 

 the shore beds. In these cases the beds have been moved but little since 

 their deposition. 



(2) A slight excess of sinking on the eastern side or an increment in 

 the strength of the eastern currents, or both, by which the finer central 

 beds were in their eastern portion encroached upon and covered by a 

 broad transgression of the eastern conglomerates, so that all down the east 

 side the fine-grained beds dip normally beneath the coarse. 



(3) Later tilting, largely to the east, but bending to the south in the 

 Holyoke range, and generally of the monoclinal type, the impoi'tant excep- 

 tion being at the mouth of Millers River, where there is a great syncline 

 whose axis pitches sharply a few degrees south of west. By this later 

 monoclinal tilting the covered bed of fucoidal sandstone is brought up 

 several times in the mass of Mount Toby. 



A third period of volcanic activity occurred in the southern basin 

 about the time of the close of sedimentation and the final tilting of the 

 sandstones. Nearly a score of volcanoes formed a chain running from the 

 Belchertown ponds first west to the Connecticut River and then south to 

 Holyoke, apparently caused by the reopening- of weak points along the 

 great fissures which had supplied the material of the earlier sheet. One of 

 these shows indications of having been a laccolith sending out a long 

 fissure-filling in the sandstone. Another is diabase filled with gi-anitic 

 inclusions. The rest are small plugs. 



The final tilting was much more severe in the northern portion of the 



