PART 11. CHAPTER XXL 



261 



Rate of Deposition of the Coal Strata. 



extremity by penetrating to the depth of a foot or two into soft 

 mud. How long such trunks, if constantly submerged, might 

 resist decomposition, is a question which cannot, perhaps, be 

 determined ; but, judging from the duration of wooden piles 

 constantly covered by water, and trees naturally submerged, like 

 those in Louisiana,* we may conclude that they might endure 

 for many years, so that their envelopment in strata, like those 

 of the Coal, may have been effected without a very rapid rate 

 of deposition. 



If, however, we assume that strata 30 or 40 feet thick were 

 often thrown down in a few years, months, or even days, this 

 fact affords no ground for calculating the time required for the 

 formation of a wide coal-field. 



Suppose, for example, the structure of a coal-field always re- 

 sembled that exhibited in the annexed section (Fig. 272.), we 

 might then infer, that if the lowest set of strata, a, having a 

 thickness of fifty feet, required half a century for its accumula- 

 tion, the strata, a, c, constituting the entire coal-field, and being 

 150 feet thick, might have been completed in a century and a 



Fig. 272. 



a 



half. But as the beds are wedge-shaped, and often thin out ; and 

 as the successive beds of a single coal-field are usually arranged 

 in the form of a, c, d, e (Fig. 273.), we cannot calculate their 



Fig. 273. 



number from considering any one section. The deposits, a, b, 

 c, d, e, traced in a given direction, may have taken each fifly 

 years for their deposition ; but they may have been as limited in 

 breadth as in length. They may have constituted originally a 

 narrow strip of land like part of the delta formed by the Missis- 

 sippi, since New Orleans was built, by the incessant discharge 

 of mud and drift timber into the Gulf of Mexico. Although by 

 this means a narrow tongue of land has been made to protrude 

 for several leagues into the sea, yet thousands of years may 



* See Principles of Geology, Index, " Bislineau." 



