378 PALEOZOIC SYSTEM OF ROCKS. 



tins coal-basin is given on page 350 as 18,000 square miles; but the 

 identity in character of portions now widely separated by seas — e. g., 

 on Prince Edward's Island, Cape Breton, Magdalen Island, etc. — plainly 

 shows that all these are parts of one original field, which could not 

 have been less than 36,000 square miles. The thickness at South Jog- 

 gins is 13,000 feet. At Pictou, 100 miles distant, it is nearly as great. 

 We shall certainly not err on the side of excess, therefore, if we take the 

 average thickness over the whole area as 7,500 feet. This would give 

 the cubic contents of the original delta deposit as about 51,000 cubic 

 miles. Now, the Mississippi Eiver, according to Humphrey and Abbot, 

 carries to its delta annually sediment enough to cover a square mile 

 268 feet deep, or nearly exactly one twentieth of a cubic mile. There- 

 fore, to accumulate the mass of sediment mentioned above would take 

 the Mississippi about 1,000,000 years. 



It may be objected to this estimate that it is founded on a particu- 

 lar theory of the accumulation of the Coal-measures. The answer to 

 this is plain. Any other mode would only extend the time, for this 

 mode is more rapid than any other. Again, it may be objected that 

 we have evidence of a very rapid accumulation in stumps and logs and 

 erect trunks, either bituminized or petrified, and which, therefore, must 

 have been completely buried before they could decay. The answer is, 

 that these are only examples of local rapid deposit, and do not at all 

 affect the general result. Precisely the same happens now in river- 

 deltas. Again, it may be objected that the agencies of Nature were far 

 more energetic then than now. This objection has already been an- 

 swered on page 277. 



We, therefore, return to our estimate with increased confidence 

 that it is far within limits. But the Coal period, as already said (p. 

 346), is not more than one thirtieth of the recorded history of the 

 earth; beyond which, again, lies the infinite abyss of the unrecorded. 



Physical Geography and Climate of the Coal Period. 



Physical Geography. — In the eastern part of the American Conti- 

 nent the area of land during this period is approximately shown in the 

 map (p. 291). It included the Laurentian, the Silurian, and Devonian 

 areas, during the whole age. In the sub-Carboniferous period the sub- 

 Carboniferous and Carboniferous areas were covered by the sea, but in 

 the Carboniferous period proper the sub- Carboniferous area was land, 

 and the Carboniferous area, as already seen, was in an uncertain state, 

 sometimes above and sometimes below the sea-level. It is probable, 

 also, that the Eastern border-land extended then much beyond the line 

 of the Tertiary deposits (see map, p. 291), and even beyond the present 

 coast-line (see map, Fig. 266), and was partly submerged in the eleva- 

 tion of the Appalachian chain, at the end of the Coal period. 



