282 BULLETIN OF THE 



5. GENERAL DISCUSSION. 



Origin of the Triassic Estuaries. — The small number of Triassic 

 trough-deposits and their absence from the western half of our Eastern 

 mountains go to show that in the making of the Appalachians the syn- 

 clinals were not, as a rule, absolutely depressed, but on the contrary 

 took part in the general elevation, and rose with the rest of the strata, 

 although to a less height than the anticlinals : as they were continually 

 rising above drainage level, they very seldom or never served as troughs 

 for the accumulation of deposits. But, on thp other hand, the estuaries 

 or troughs in which the Triassic strata were deposited must have been 

 absolutely depressed below their previous levels ; and it seems reason- 

 able to suppose that the remarkable relation existing between the trap 

 and sandstone areas, so often alluded to, must be mechanically depend- 

 ent on this downfoldiug or absolute depression of the estuaries ; for 

 here alone where there is evidence of absolute local depression are the 

 only post-carboniferous eruptions of trap to be found from the Green 

 Mountains to Alabama. That this may be truly a relation of cause 

 and effect is made the more probable by the evidence given in the fol- 

 lowing pages that much of the trap, if not all, was ejected during the 

 downfolding and filling in of the troughs. 



Professor Dana {e, 113) considers that the subsidence which ended in 

 the post-Triassic eruptions was slow, and not more than five thousand 

 feet, and that it caused in the end only small displacements of strata, 

 wholly inadequate to cause the fusion and ejection of deep-lying rocks 

 from which the traps were derived. He further instances the Green 

 Mountains as a region where the folding was much stronger, and yet 

 where no eruptions but only metamorphism took place, and takes this 

 as arguing against the possibility that the moderate Triassic disturbance 

 was the cause of the fusion and ejection of the traps. 



However it may be with the fusion, I must differ from this opinion 

 concerning the eruptions ; it seems best, in view of what has been stated 

 above, to suppose that the Triassic disturbance was directly and cau- 

 sally connected with the fact and act of the eruptions. The occurrence 

 of absolute depression in the slightly disturbed Triassic troughs seems 

 reason enough for mechanical eruptions taking place here, although 

 absent elsewhere in regions of greater disturbance but general absolute 

 elevation. 



