PALEOZOIC TIME — CAEBONIC. 715 



Tvise Archgean confines, even during Cambro-Silurian time, each having 

 been an independent trough or basin. In the Acadian trough the subsidence 

 carried down the bottom of the trough as deposition went forward, but not 

 the Archaean ridges along the confines ; for if these Archaean ridges had sub- 

 sided also, they should have had, at the beginning, the extremely improbable 

 height of 30,000 or 40,000 feet. The Acadian and the Gaspe- Worcester 

 troughs were sinking, and receiving, in some parts, if not generally, formation 

 after formation, to the close of the Carboniferous period ; and the Connecticut- 

 valley trough, to the middle or later part of the Devonian era ; and this was 

 not the last, as will be shown, of the rock-making carried on in the Acadian 

 and Connecticut-valley troughs. ' The western part of the Continental Sea 

 had also its areas of subsidence and deposition. Only subsiding troughs 

 received thick deposits for the various formations. 



2. Diversities in kinds and in thickness of rocks. — The vast Continental 

 Interior, shut away from the more destructive forces of the ocean, afforded 

 the most favorable conditions possible for the growth of aquatic life, and 

 therefore for the making of limestones ; and the life had no doubt the 

 luxuriance prevailing in the existing coral reef seas of the tropics. 

 What this degree of luxuriance is at the present time may be well 

 learned from the admirable photographs of a volume by W. Saville Kent on 

 Tlie Great Barrier Reef of Australia. To see the reefs themselves is better; 

 but this not being readily attainable, the geological student, who would ap- 

 preciate the profusion of life, and something of the beauty of Paleozoic reef- 

 grounds, should see the photographs. The colors are absent, but there is 

 everything else in the pictures. The species represented are modern Corals 

 of various kinds and forms ; but it will be easy, afterward, to think of vast 

 areas of Crinoids, ancient Corals, and other Paleozoic productions ; for the 

 result is the same in kind, if shell-making Mollusks were the chief kind of 

 life. He would learn also the pertinent fact that limestone-making is not 

 necessarily, or ordinarily, deep-water work. 



The effects of the tidal and wind-made currents in forming fragmental 

 accumulations within the Interior Sea, especially along its borders, have been 

 variously illustrated in the preceding pages, with special reference to those 

 of the northeast and east ; and there has been brought out to view, also, the 

 contrast with those of the limestone formations over its interior. This 

 contrast was augmented through each of the successive periods by the con- 

 trast in the amount of subsidence in progress : — over the Interior Sea, but 

 little, the formations only 3000 to 6000 feet thick ; over the eastern portion, 

 a great subsidence, 30,000 to 40,000 feet, because included within the area of 

 the subsiding Appalachian trough. In the Continental Interior, the Paleozoic 

 rocks are full two thirds limestones. The coal formation there has many 

 limestone strata ; the Subcarbonif erous consists mostly of limestone ; the 

 Devonian and Upper Silurian strata are chiefly limestone ; the Lower Silurian, 

 even through the Hudson period, mostly limestone ; and the Cambrian 

 chiefly limestone. The intercalations of strata of sandstone and shale indicate 



