136 THEORY OF THE EARTH, 



sphagna and other aquatic mosses, also afford a 

 measure of time. They increase in height in 

 proportions which are determinate with regard 

 to each place. They thus envelope the small 

 knolls of the lands on which they are formed ; 

 and several of these knolls have been covered over 

 within the memory of man. In other places the 

 peat-mosses descend along the valleys, advancing 

 like glaciers, but differing from them in this re- 

 spect, that, while the glaciers melt at their lower 

 part, the progress of the peat is impeded by no- 

 thing. By sounding their depth down to the so- 

 lid ground, we may estimate their age ; and we 

 find, with regard to these peat-mosses, as with re- 

 gard to the downs, that they cannot have derived 

 their origin from an indefinitely remote period. 

 The same observation may be made with regard 

 to the slips or fallings, which take place with won- 

 derful rapidity at the foot of all steep rocks, and 

 which are still very far from having covered them. 

 But as no precise measures have hitherto been ap- 

 plied to these two agents, we shall not insist up- 

 on them at greater length *. 



* These phenomena are very well treated of in M. Deltic's 

 Letters to the Queen of England, in the parts where he de- 

 scribes the peat-moses of Westphalia ; and in his Letters to 

 Lametherie^insertedmthe Journal de Physique for 1791,&c. 

 as well as in those which he has addressed to Blumenbach. 

 We may refer also to the very interesting details which 



