708 HISTORICAL GEOLOGY. 



GEOLOGICAL AND GEOGRAPHICAL CHANGES DURING THE PROGRESS OF THE 



COAL-MEASURES. 



The beds of the Coal-measures vary in kind of rock between shales, sand- 

 stones, conglomerates, and limestones, clay beds, iron ore beds, and coal-beds ; 

 and differ in conditions of origin, between those of salt water, brackish 

 water, and fresh water. Moreover, the beds bear evidence of the changes in 

 water level that took place during the progress of the long series. In the 

 various regions, the clayey beds beneath the coal evince that they were 

 usually of marsh or fresh-water origin, like the coal-beds, by the absence of 

 marine relics, and the presence of roots and sometimes of stumps of the 

 trees that grew in the clay as their soil. 



In Nova Scotia, where deposits were made during the era to a thickness 

 of 13,000 feet, the beds of the Subcarboniferous are partly marine, but the 

 Coal-measures and Permian are mainly of brackish or fresh-water origin; 

 for only one bed has been found to contain marine fossils. This region was 

 a wide basin in the Acadian trough, at the mouth of the St. Lawrence River. 

 Specimens of the Pupa or land-snail, described by Dawson (page 676), occur 

 in an under-clay more than 1200 feet below the level of the stump in which 

 the species was first discovered ; and in this interval there are 21 coal-seams, 

 showing, as Dawson observes, that the species existed during the growth and 

 burial of at least 21 forests. 



The oscillations in water level, indicated by the alternations in the 

 deposits, were slow in progress ; movement by the few inches a century 

 accords best with the facts. When under verdure, the surface must have 

 lain for a long period almost without motion ; for only a very small change 

 of level would have let in 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. Again, for the making of shales or sand- 

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

 swept by the waves and currents, subsiding with extreme slowness, so as to 

 make thick deposits without letting in the sea. It may have been long a 

 region of barren marshes, and, in this condition, have received its iron-ore 

 deposits, 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 holding marine fossils. 

 Again the land slowly emerged 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. Uniformity in oscillations of level, through so great 

 an area, is not probable ; and therefore the former continuity of a single 

 coal-bed through the East and West requires strong proof to be admitted. 



Such alternations of verdure and rock depositions occurred also during 



