566 STEVENSON— FORMATION OF COAL BEDS. [November 3. 



ical streams, are prevailingly allochthonous. In this last his refer- 

 ence is to the record cited by Lyell,^^ of a boring in the Ganges delta, 

 which passed through a deposit, having certainly the characteristics 

 of a peat bed. In view of what has been said on preceding pages 

 respecting the accumulation of drifted vegetable matter and of the 

 fact, that the great deposits in the Mississippi delta, formerly sup- 

 posed to be of drifted material, are of in situ origin, one is justified 

 in saying that the testimony of that and other borings cannot be 

 waived aside lightly by the mere assertion of allochthony. The onus 

 of proof is on the one making the assertion. 



It is difficult to understand why the a priori reasoning that trop- 

 ical heat should prevent peat formation is thought so conclusive as 

 to make worthless the testimony, which would be accepted as prov- 

 ing the presence of peat in Michigan or north Germany. The con- 

 ditions of temperature during summer in much of the United States 

 are decidedly tropical ; yet peat accumulates. It is true that vege- 

 table matter exposed to moist air must decay more rapidly where 

 the temperature is constantly high than in the temperates, where the 

 hot period is of brief duration ; but that has nothing to do with the 

 matter under consideration, for one is concerned with decay of vege- 

 table matter protected from atmospheric action by a plentiful cover 

 of water. A priori^ one should expect to find peat accumulating in 

 those tropical regions, where the conditions are such as encourage 

 peat-making in the temperates — with only the difiference that, owing 

 to the continuous high temperature, complete decomposition should 

 be more rapid and the bog should have the vegetable mud near the 

 surface. But one is not dependent on a priori reasoning. 



Harper''^ notes that it is an error to suppose that peat is confined 

 to cold climates, since high temperature does not prevent its forma- 

 tion if humidity and topography favor. Peat is abundant on the 

 very border of the tropics in Florida, where tropical temperature 

 prevails throughout the year. Sphagnuui does not occur south from 



"^ C. Lyell, " Principles of Geology," Eleventh Ed., New York, 1872, 

 Vol. I., p. 476. 



°*R. M. Harper, "Preliminary Report on the Peat Deposits of Florida," 

 3d Ann. Rep. State Survey, iQio, pp. 214, 274, 287, 292. 



164 



