710 HISTORICAL GEOLOGY. 



eighth of an inch thick corresponds to an inch, at least, of the accumulating 

 vegetable remains ; and hence the regularity and delicacy of the structure 

 are not surprising. Alternations are a consequence of (1) the periodicity in 

 the growth of plants and the shedding of leaves ; (2) the periodicity of the 

 seasons, the alternations of the season of floods with the season of low 

 waters or comparative dryness ; (3) the occurrence, at intervals of several 

 years, of excessive floods. Floods may bring in more or less detritus, besides 

 influencing the fall and distribution of the vegetation. There may have been 

 great variations in the length of time before the peat-like vegetation after 

 its formation was put under the pressure of beds of clay or sand ; and the 

 precise quality of the coal would be varied thereby, the decomposition of the 

 vegetation depending on the amount of water, the composition of that water, 

 and the length of time exposed. 



In some parts of the marshes there were pools or lakes where the vege- 

 tation was long steeping and so becoming reduced to a pulp, to the oblitera- 

 tion of all bedding ; and in such places, according to ISI ewberry, cannel coal 

 was often formed; for it usually constitutes locally the lower parts of a coal- 

 bed, though sometimes making the whole thickness. And, as such ponds or 

 lakes were likely to have their living species, so a bed of cannel coal often 

 contains remains of fossil Fishes, Eurypterids, Crustaceans, and other species. 

 The Eurypterus in its bed of Eerns figured on page 677 was obtained from a 

 locality of cannel coal. 



In conclusion, the Coal period was a time of unceasing change, — eras of 

 verdure alternating with others of wide-spread waters, destructive of all the 

 vegetation and of other terrestrial life except that which covered regions 

 beyond the Coal-measure limits. Yet it was an era in which the changes 

 went forward for the most part with such extreme slowness, and with such 

 prevailing quiet, that, if man had been living then, he would not have sus- 

 pected their progress. 



In Europe the conditions were similar, in the main, to those of America. 

 The succession of Carboniferous rocks and coal in the British Isles exceeds 

 much in thickness that in any part of Europe, very much as that of Nova 

 Scotia exceeds that of Pennsylvania and the states west. The greater thick- 

 ness of the formations (if not of the coal-beds), supposing the peat-making 

 conditions to exist, has probably depended in each region on the extent of 

 the slowly progressing subsidence or geosyncline. The longer continued and 

 deeper subsidence in Nova Scotia favored greater thickness than in Pennsyl- 

 vania; and the amount of subsidence in Pennsylvania determined greater 

 thickness in that state than in Illinois. So it was also in the British Isles 

 as compared with Europe. Far west of the Mississippi in North America 

 the general submergence of the surface put a Carboniferous limestone over 

 the region instead of profitable Coal-measures ; and far east in Europe, Russia 

 has her barren coal-strata of vast extent, on both sides of the Urals. 



For the making of extensive Coal-measures a nice balancing of the land 

 surface between submergences and emergences was a requisite. With a very 



