LOWER SILURIAN. 211 



12,000 feet (Rogers); in the Green Mountains, not less; in Canada, 

 north of Lake Champlain and Vermont, at least 7,000 feet ; in East 

 Tennessee, 15,000 feet, or more. 



(c.) Proportion of limestones to the sandstones and shales less in the 

 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 Dutchess 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, fragmental rocks, — rocks of sand, pebbles, mud, and clay 

 — ought to have abounded. The interior basin, under the protection 

 of this barrier, was occupied by relatively quiet seas, and fitted there- 

 by 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. 



(e.) The Appalachian region experienced through the Lower Silurian 

 greater changes of level than the Continental Interior. — If the Appal- 

 achian region was an area of comparatively shallow waters, or the 

 course of a great, though mostly submerged,continental barrier, as just 

 stated, it follows that there must have been a gradual sinking of the 

 bottom, in order that the depositions should have reached the great 

 thickness, in different parts, of 10,000 to 20,000 feet. For only by 

 such a subsidence could the accumulations have exceeded in thickness 

 the actual depth. It must have been an extremely slow subsidence, 

 not faster, on the average, than the rate of progress in the deposi- 

 tions. The succession of different kinds of rocks, — sandstones, 

 shales, conglomerates, limestones, — shows that the sinking went on 

 interruptedly, or was the resultant after a long series of oscillations, 

 in which the surface was here and there at times emerged. 



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 



