Dr MacCulloch on Feat. 5S 
of operations which takes place in lakes, and thus the cavity be- 
comes once more filled with the same material, always, however, 
distinguishable for a long period by the laxity of its texture. If 
the exposed surface is drained, no such event follows, but the 
spot appears to remain for ever naked, and of course incapable 
of augmentation or renewal, as may daily be witnessed in moun- 
tains, where the perfectly formed and compact peat has been either 
naturally or artificially exposed. Thi§ is now well understood ; 
and it has accordingly become a general practice to reserve the 
superficial covering, and replace it on the bottom, after the peat 
required for fuel has been removed. 
The time required for the production of a given depth of 
peat has been a frequent subject of inquiry. In some cases, its 
growth has been found so rapid as to be sensible to the obser- 
vation of individuals ; but, in general, it has been necessary 
to have recourse to evidence founded on circumstances invol- 
ving a mucb longer period of time than human life. Such re- 
gisters of the date of given depths of peat have been found in 
works of art, or in the marks of human industry which have 
been submerged in it, and of which the time admitted of being 
ascertained within certain limits. From these several modes of 
judging, it has appeared, that the time required for the format 
tion of a given depth of this substance, is so various, that no 
general conclusion can be drawn on the subject. It has been 
remarked on the Continent of Europe, that cavities of seven feet 
in depth have been filled with it in the short space of thirty years. 
This occurrence is, however, limited to partial spots, and de- 
pends on a combination of favourable circumstances, which must 
be considered rare. It only takes place in the smaller excava- 
tions, in which the vegetation of aquatic and marsh plants is 
unusually luxuriant. Two cases have often been quoted^ of 
peat-mosses formed on fallen forests, the one at Drumlanng, the 
other near Loch Broom, the former of which was completed in 
sixty, the latter in fifty years; but as the depth is not specified, 
no conclusion can be drawn from these instances. In innumer- 
able other instances, Roman works of art, and traces of Roman 
roads, have been found beneath peat-mosses, but the depth has 
seldom been measured ; so that from these also, no conclusion 
respecting the quantity produced in a given time, can be drawHo 
