320 LECTURE XI. 



sical changes of tlie surface^ and the transposition of clay, 

 gravel, sand, etc., require long periods of time. 



Every person who carefully examines the enormous accumu- 

 lations which the glaciers and the rivers, sprung from them, have 

 deposited upon Swiss soil, must admit that only a long series 

 of centuries, the number of which can scarcely be estimated, 

 could have effected them. This assertion may be proved by 

 the estimation of some individual factors of the beds. We 

 have seen that the coal bed of Diirnten forms but a moderate 

 intermediate stratum in the so-called diluvial beds. This 

 stratum at its greatest thickness, reaches about twelve feet, or 

 rather, ten, for in many places there are intermediate seams of 

 letten. " This, its greatest thickness," says Heer, " may 

 enable us to calculate the period of time requisite for the for- 

 mation of this bed. From the mode and manner of the com- 

 pression of the stems of the trees, from a comparison of the 

 carbonaceous constituent of slate-coal with that of the peat, it 

 follows that these coal beds, when in the condition of peat, 

 ought to have been six times thicker, that, consequently, the 

 above coal-bed of ten feet thickness must have originated from 

 the compression of a peat-bed sixty feet high. Assuming, on 

 the average, an increase of one foot of peat within a century, 

 we obtain 6,000 years. 



''Another mode of calculation leads to the same result. 

 An acre of slate-coal ten feet thick contains, according to 

 Mine- Superintendent Stokar-Escher, 96,000 hundred-weights 

 of carbon. On assuming that an acre of peat-land produces 

 annually fifteen quintals of carbon, 6,400 years would be 

 required for the formation of the above-mentioned coals. The 

 assumption of an annual production of fifteen quintals of car- 

 bon (based upon the assumption that a foot of peat is formed 

 within a century), is rather too high than too low, for accord- 

 ing to Liebig's interesting researches, an acre of wood plantation 

 produces annually but ten quintals of carbon, in which case the 

 formation of the above coal-bed required 9,600 years.^^ 



In these calculations it is certainly assumed that the climatic 

 conditions resembled those at present existing. As the same 

 species of plants, which now produce peat, formed the slate-coal. 



