382 J. D. Dana — Archcean axes of Eastern N. America. 



The troughs are areas, for the most part, of independent 

 geological work. — It is of great geological interest that subse- 

 quent to the Lower Silurian era, if not also before, these 

 troughs were, for the most part if not wholly, areas of inde- 

 pendent geological work, and that in them all the geological 

 formations of this part of North America, up to and including 

 the Jura-Trias, were made. This is apparent from the facts 

 respecting the rocks and fossils of the troughs already men- 

 tioned. 



The thickness of the Silurian and Devonian rocks of the 

 Connecticut Valley trough is not known ; but it is certain that 

 the Jura-Trias rocks of the southern half of the valley have a 

 thickness of at least 5000 feet ; and this proves that subsidence 

 to this large amount took place in the valley at so late a date 

 in geological history. 



The Acadian trough has its thick Silurian and Devonian for- 

 mations yet unmeasured ; and then, some 16,000 feet of Car- 

 boniferous rocks, and later, a great thickness of Jura-Trias — 

 as great probably as in the Connecticut valley ; moreover the 

 Newfoundland part of the trough has, according to estimates 

 made, some 6000 feet of Carboniferous beds near St. George 

 Bay. 20,000 feet of subsidence is thus recorded for parts of 

 the Acadian trough after the Devonian era. And probably as 

 much tdok place before the Carboniferous era. 



The other troughs bear evidence of like subsidence and 

 work ; but we have no definite estimates as to the thickness of 

 the deposits. 



Further, we find that the Connecticut valley trough was in 

 its southern half, through a long period, that of the Jura- 

 Trias, a fresh or brackish water trough. Again, the Gaspe- 

 Worcester trough became a trough of fresh-water marshes in 

 its southern part during the Carboniferous era. The Acadian 

 trough, passed from a salt-water channel to a condition of great 

 fresh-water marshes and estuaries after the Carboniferous 

 period had set in ; and this may possibly have been its con- 

 dition for a length of 800 miles. It was while in this state 

 that the trough in part of Nova Scotia sunk 18,000 out of the 

 16,000 feet. And after the Carboniferous era of fresh-water 

 marshes, alternating with fresh or brackish water estuary con- 

 ditions, during deep subsidence, had passed, and after also the 

 mountain-making events which closed Paleozoic time in Nova 

 Scotia as well as the Continental Interior, the trough still ex- 

 isted and in some parts continued its work through another 

 long period of deep deposition and subsidence, — that of the 

 Jura-Trias.* 



* The broad margin of the continent south of New York had its troughs in the 

 Jura-Trias. But each of the troughs contains Jura-Trias rocks alone; the beds 

 rest on the crystalline Archaean, or on Lower Silurian rocks crystalline or uncrys- 



