GENERAL OBSERVATIONS. 391 



laehians of Pennsylvania, and consist of sandstones, shales, and conglomerates 

 with little limestone ; in those of Virginia, the limestones are of great thick- 

 ness : the formations cover the Interior basin, and are almost wholly of lime- 

 stone, the lower part only being arenaceous or siliceous. 



The Carboniferous Millstone grit is a conglomerate of great extent in the 

 Appalachians, mostly a sandstone in Ohio, and is nearly wanting in the Mis- 

 sissippi basin. The Coal measure beds are shales and sandstones with little 

 limestone in Pennsylvania and Ohio ; shales and sandstones with considerable 

 limestone in the Mississippi basin east of Kansas ; and mainly limestone west 

 of Kansas. 



This brief sketch of the limits of the formations gives an idea of the great 

 oscillations in the sea-level during the progress of the Palaeozoic : it is not 

 necessary to dwell upon its details, as the conclusions may be readily drawn by 

 the reader after the explanations, under the head of General Observations, in the 

 preceding pages. The Eastern border region has here been left out of view. 



Until the close of the Subcarboniferous period, the oscillations 

 had that wide continental range which was eminently characteristic 

 of the American Palaeozoic. In the period following, the Carbo- 

 niferous, the continent for prolonged periods stood raised just 

 above the ocean, at a nearly uniform level, — so low that its inte- 

 rior was covered with immense fresh-water marshes, and for so 

 long eras that the vegetable accumulations attained the thickness 

 sufficient for coal beds (p. 360) ; but these emergences had their 

 alternation with submergences. The system of oscillations, though 

 slower in movement, was still continued ; yet the movements were 

 less general ; and it is therefore difficult to make out a parallelism 

 in the beds of coal and intervening rock-strata through the East 

 and West. 



3. Uplifts and dislocations. — The only mountain -region along the 

 course of existing chains which can now be pointed to as having 

 emerged during the Palseozoic ages, is that of the Green Mountains. 

 This region probably became part of the stable dry land in the in- 

 terval between the Potsdam period of the Lower Silurian and the 

 commencement of the Upper Silurian er«*. 



The crystalline limestones of western New England have been 

 referred to the Calciferous epoch. They extend from Connecticut 

 through Massachusetts (in which State the Stockbridge quarries 

 are most noted), and nearly to the northern limit of Vermont ; 

 and in Vermont, where they have been called " Eolian" limestones, 

 they have, according to Hitchcock, a thickness of 2000 feet. If 

 actually Calciferous, the emergence of the Green Mountain region 

 may have taken place before the Trenton period, since no Trenton 

 rocks in that case were formed over it. The fossils ha^e in general 

 been obliterated by the crystallization of the rock. Evt th^ Ver- 



