CARBONIFEROUS AGE. 357 



other inland species ; in the other, full of Crinoids and Brachiopods, 

 the life of the sea. The great extent of the continent, wherever these 

 strata occur, underwent, therefore, continued oscillations of level, or 

 the sea as unceasing changes of water-level. After a period of verdure, 

 there followed a desolation as complete as that when the subjacent 

 Millstone-grit was spread over the surface, — either a subsidence of 

 the interior, or some other change, that led to a general submergence 

 beneath fresh-waters, or a similar subsidence, or else a removal or 

 sinking of barriers, that placed the whole beneath salt water ; in either 

 case, the former vegetation gave way to aquatic life again. 



The broken relics, that were a result of the catastrophe, are often 

 packed together in the first deposits that ensued. Lesquereux states 

 that, in the roof-shale of the coal-bed at Carbondale, Pa., there was 

 found an impression of the bark of a Lepidodendron, two feet wide 

 and seventy-jive feet in length. Andrews mentions that thousands of 

 the trunks of the Fern, Pecopteris arborescens Lsqx., are found in the 

 shale over the Pomeroy Coal-bed ; and that at one place the trunk of 

 a Sigillaria was traced by him for more than forty feet. 



The oscillations must have been exceedingly various, to have pro- 

 duced all the alternations of shales, sandstones, limestones, and ore- 

 beds. 



The movements, moreover, must have been slow in progress : mo- 

 tion by the few inches a century accords best with the facts. When 

 under terrestrial vegetation, and receiving vegetable debris for coal- 

 beds, it must have lain for a long period almost without motion ; for 

 only a very small change of level would have let in the salt water to 

 extinguish the life of the forests and jungles, or have so raised the 

 land as to dry up its lakes and marshes. Hence the grand feature of 

 the period was its prolonged eras of quiet, with the land little above 

 the sea-limit, — a condition that made coal-beds also in later geo- 

 logical ages. Again, for the making of the shales or sandstones, 

 the continent may have rested long near the water's surface, just 

 swept by the waves. It may have been long a region of barren 

 marshes ; and, in this condition, it might have received its iron-ore de- 

 posits, as now marshes become occupied by bog-ores. It must have 

 been long in somewhat deeper waters, and covered with a luxuriance 

 of marine life, in order to have received its beds of limestone. Finally 

 the land slowly emerged again from the waters, and the old vegetation 

 spread rapidly across the great plains, commencing a new era of coal- 

 making vegetable debris ; or the escape was only partial, and coal- 

 plants took possession of one part, and made limited coal deposits, 

 while the sea still held the rest beneath it : for uniform oscillations of 

 ievel in all cases, through so great an area, are not probable ; and 



