LOWER SILURIAN. 



211 



Appalachian region and to the north, than over the Interior basin. — 

 Out of the whole thickness of the rocks in Missouri and Illinois, 

 five-sixths are limestone, and in Iowa, one-half. In the Appalachian 

 region, out of the 12,000 feet, 5,000 feet, or five-twelfths, are lime- 

 stone, according to Rogers ; in Tennessee, at least one-third ; in 

 Canada, about Quebec, not one-twentieth. ' 



(d.) The Appalachian region, the Green Mountains included, from 

 the period of the earliest Silurian, a region of comparatively shallow 

 waters. — Along its course, there were Archaean islands and reefs, 

 when the Silurian era opened, — portions of the Blue Ridge to the 

 south, the Highlands of New Jersey and Orange and Putnam Coun- 

 ties, N. Y., and the patches of Archaean rocks in New England being 

 some of these areas. It was hence a barrier region to the continent, 

 over which the Atlantic currents flowed and waves broke ; and here, 

 therefore, f ragmental rocks — rocks of sand, pebbles, mud, and clay — 

 ought to have abounded. The interior Continental basin, under the 

 protection of this barrier, was occupied by relatively quiet seas, and 

 fitted thereby for the growth of Crinoids, Corals, and Mollusks, whose 

 calcareous relics were the material of the limestones. This point is 

 illustrated by nearly all the successive formations. 



2. General quiet of the Lower Silurian era; Limited disturb- 

 ances. — The strata of the Lower Silurian in North America appear 

 to have been spread out over the Interior Continental basin in hori- 

 zontal beds of great extent, and to have followed one another without 

 much disturbance of the formations. There were extended oscillations 

 of the surface of the continent; for this is indicated in the varying lim- 

 its of the formations, as well as the alternations in the kinds of rocks. 



One marked exception to the general quiet occurred during some 

 part of the Canadian period, in the region of Lake Superior, where 

 there were extensive igneous ejections (p. 185). Another case of dis- 

 turbance has been noted in Newfoundland (p. 182). But still it re- 

 mains a fact that the Lower Silurian was an era of comparative quiet. 

 This quiet, moreover, was a long one, —probably as long as all of the 

 time that has since elapsed. 



5. DISTURBANCES AT THE CLOSE OF THE LOWER SILURIAN. 

 The long quiet was finally interrupted, in some parts of the conti- 

 nent, by subterranean movements and metamorphism, — not by sudden 

 catastrophe, but, after the ordinary style in geological progress, by 

 slow and gradual change. The principal regions of this change, now 

 known, are that of the Green Mountains, the northern extremity of 

 the Appalachian region, and that of the " Cincinnati uplift," from Lake 

 Erie, over the Cincinnati region, into Tennessee. 



