212 Prof. J. D. Dana on some Results of 



The under-Appalacliian fire-sea, if a reality, must hence have 

 had a long continuance. 



But on the above condition it could not have begun its ex- 

 istence later than the period of disturbance closing pre-Silurian 

 time. Earlier great subsidences were involved in the deposition 

 of the material of which the Blue Bidge, Highland Bidge, 

 Adirondack s, and the Archaean heights further north were made ; 

 and the undercrust sea would have been through all a necessity. 

 In fact it is difficult to find a reason for doubting its having 

 dated back to the era of general fluidity. 



Directly following the Palaeozoic, or as its closing event, as 

 explained on a preceding page, occurred the plicating of the 

 Alleghany rocks to their depths miles below, and the crystalli- 

 zation of part of them ; and this epoch ended in the making of 

 the mountains (a synclinorium) and the annexation of the central 

 and western part of the region to the essentially stable area of 

 the continent ; and if motion in the rocks was ever transformed 

 into heat, the under-Appalachian sea should have had its tem- 

 perature, or its extent, or both, increased. Then, after the Ap- 

 palachian region had thus become essentially stable, the locus of 

 the region of yielding was moved to the eastward. The long 

 range of the Triassico-Jurassic beds, on the Atlantic border from 

 Nova Scotia to southern North Carolina, show the positions of 

 the new troughs, as stated at page 133. These subsidences, 

 amounting to 4000 feet in some parts, ended in a tilting of the 

 beds and in fissure eruptions through all these sandstone regions 

 from the most northern to the most southern. Now the ques- 

 tion arises whether the great under-Appalachian fire-sea of the 

 Palaeozoic continued on through the Triassic and Jurassic periods 

 of the Mesozoic, and thus favoured the subsidences and erup- 

 tions that then took place — or whether the old sea of viscous 

 rock, after being increased in extent or temperature by the pro- 

 found plicating and faulting of the Appalachian revolution, then 

 ceased to exist (in some way difficult to understand), and others 

 were made further east by the later movements. Such a ceasing 

 with a subsequent renewal is seemingly improbable ; and if it did 

 not occur, then the under-Appalachian fire-sea continued from, 

 the Palaeozoic far into the Mesozoic era. 



When the material of the under-Appalachian sea was pushed 

 aside by the subsiding Palaeozoic deposits of the Appalachian 

 region, what became of it ? Some of it may have moved off 

 southward. The chief part would pass either to the west 

 or to the east. That it did not go west is evident from the 

 ascertained fact that the oscillations in that direction during 

 Palaeozoic time were small; for the region was then the larger 

 part of the time a mediterranean salt-water basin or sea, nur- 



