320 



J. D. Dana on American Geological History. 



and Lower Silurian, — another proof of the violence that closed • 

 the Lower Silurian era.* 



But let us pass onward in our geological record. 



All the various oscillations that were in slow movement 

 through the Silurian, Devonian, and Carboniferous Ages, and 

 which were increasing their frequency throughout the last, rais- 

 ing and dipping the land in many alternations, were premoni- 

 tions of the great period of revolution, — so well elucidated, as 

 already observed, by the Professors Eogers, — when the Atlantic 

 border, from Labrador to Alabama, long in preparation, was at 

 last folded up into mountains, and the Silurian, Devonian, and 

 Carboniferous rocks were baked or crystallized. No such event 

 had happened since the revolution closing the Azoic Period. 

 From that time on, all the various beds of succeeding ages up to 

 the top of the Carboniferous had been laid down in horizontal 

 , or nearly horizontal layers, over New England as well as in the 

 West, — for the continent from New England westward, we have 

 reason to believe, was then nearly a plain, either above or below 

 the water ; there had been no disturbances except some minor 

 uplifts : the deposits, with small exceptions, were a single unbro- 

 ken record, until this Appalachian revolution.f 



This epoch, although a time of vast disturbances, is more cor- 

 rectly contemplated as an epoch of the slow measured movement 

 of an agency of inconceivable power, pressing forward from 

 the ocean towards the northwest ; for the rocks were folded up 

 without the chaotic destruction that sudden violence would have 

 been likely to produce. Its greatest force and its earliest begin- 

 ning was to the northeast. I have alluded to the disturbance 



* This Eastern border of the American continent, then in process of formation 

 over the present Appalachian region from Labrador and Canada southwestward, lay- 

 deeper to the south than to the north. In Canada and the Azoic of Northern New 

 York, there was land out of water, forming its northern limit. From thence it 

 stretched on with its gradually deepening waters, though varying constantly- with 

 the oscillations. The thickness of many of the sedimentary beds passing southward 

 from the New York Azoic prove this increasing depth to have been a general fact ; 

 and it is corroborated by a statement made by Prof. W. B. Rogers (meeting of Amer. 

 Assoc. in August last at Albany), that the subcarboniferous sandstones and shales 

 containing but little limestone in Pennsylvania, were replaced by beds of the sub- 

 carboniferous limestone which to the south in Virginia reach a great thickness 

 (see note to page 317) — the limestones indicating clearer and somewhat deeper 

 waters. The early disturbances and uplifts in the northeast near Gaspe and along 

 the Hudson valley also accord with this view. 



Again, the position of the Azoic dry land in Canada and of the sedimentary rocks 

 south and southwest, shows us that the Continent in those early times received the 

 northern Labrador current, — which would have kept by the shore as now, along the 

 eastern border of this Azoic, — over New Brunswick and Nova Scotia, and that thence 

 its natural course would have been southwest over the Appalachian region, where 

 the sandstones and shales were extensively accumulated ; and therefore its aid in 

 making these deposits can scarcely be doubted. 



f It is urged by Prof. Hall and others that the Carboniferous beds in the west lie 

 unconformably on the beds below. But the disturbance indicated was not one of 

 bold flexures or uplifts. 



