370 ON THE DURATION OT COAL IN NORTHUMBERLAND, &IC. 



the Mississippi and Missouri, are inundated, by the melting of the 

 snow near their sources, they pour down immense floods, which fill 

 their banks, and absolutely choke up the mouths of the large second- 

 ary rivers that enter them, and throw their waters back for many 

 miles, charged with the mud of the great descending waters. The 

 waters of these secondary rivers in their backward course, overflow 

 their banks, and spread over the lower parts of the level plain, form- 

 ing lakes of twenty miles or more in length : after some time these 

 lakes are gradually drained by the subsidence of the rivers. The 

 inundations are, however, prolonged by another circumstance. The 

 Missouri and Mississippi rise in different latitudes, and their periodi- 

 cal inundations do not take place at the same time. When one of 

 these mighty streams is inundated, it blocks up the passage of the 

 other, and this reacts on the secondary streams, and prolongs the 

 time of periodical inundation. Thus in these temporary lakes of 

 fresh water, we have the conditions required for the formation of fu- 

 ture coal fields — swamps promoting the rapid development and de- 

 composition of vegetables — and periodical inundations of water, 

 charged with sand and mud, to cover the vegetable beds with earthy 

 strata. It is further deserving notice, that over a large part of the 

 plain of the Mississippi, the rapid annual growth of grasses and this- 

 tles, exceeds any thing of which this part of Europe affords an ex- 

 ample : this enormous mass of vegetation perishes every winter. 



In the account of the probable duration of the coal of Durham and 

 Northumberland (page 124.), first published in the third edition, I 

 have stated the period of exhaustion to be about 350 years from the 

 present time. In evidence on the subject, given before the House 

 of Commons, the period has been extended to 1727 years; a differ- 

 ence so great as to require some animadversion. In the first place, 

 all evidence given before Parliament, by commercial or public bod- 

 ies, who have a particular object to establish, must be received with 

 considerable caution. The coal owners in the north were alarmed, 

 lest some restriction should be laid on the unlimited export of coal, 

 if it were known that the coal fields of Northumberland and Durham 

 could afford a regular supply of fuel only for a very limited number 

 of years. It was stated before the House of Commons, that there 

 are 837 square miles of coal strata in Northumberland and Durham, 

 and that only 105 square miles have been worked out. It was as- 

 sumed, that each workable bed of coal spreads under the whole ex- 

 tent of the coal fields, but this is very far from being the fact. Ma- 

 ny of the best and thickest coal beds crop out long before reaching 

 the western termination of the coal field, or are cut off by faults or 

 denudations. The thickness of the beds was also overstated. Pro- 

 fessor Buckland estimated the duration of the Northumberland and 

 Durham coal, according to the present rate of consumption, at 400 

 years, which agrees very nearly with the period of exhaustion I had 

 assigned. 



