44 ? 
Dr MacCulloch mi Peat 
portant set of changes on the surface, which consists in the obli- 
teration of lakes, and the increase of dry land. From the re- 
marks immediately preceding, it will appear, that it is intimate- 
ly connected with the generation of marsh-peat ; since, during 
its whole progress, it is attended by, and finally terminates in 
this variety. 
There are thus two distinct modes in which peat is formed in 
lakes, and, in eiich, a distinct set of plants is engaged. There is, 
however, a certain point at which these unite, and at which both 
processes are combined in producing the result by which the 
water is ultimately excluded, and a mass of solid materials is at 
length found occupying the place which it once possessed. 
In the shallow parts of lakes are found numerous subaquatie 
plants, which, in summer, flower at the surface, subsiding to the 
bottom after that process is completed. By the annual death of 
portions of these, a stratum of peat is formed at the bottom of 
the water, where this process is also constantly going on by the 
successive decomposition of the Scirpus acicularis, Lobelia dort- 
manna, Subularia aquatica, and other plants, which form a con- 
tinuous mat of verdure, where the water is sufficiently shallow 
to admit of their growth. The death of fresh-water shell-fish, 
and the deposits of earth and sand brought into the lake by 
rivers, assist still farther in shoaling the water, and in thus af- 
fording a lodgment to another class of plants, which advance 
from the margin in succession, according to their several pro- 
pensities to thrive in greater or less depths, or on the half drain- 
ed and marshy meadow by which the lake is bounded. 
In other cases, no distinct bed of peat is produced from the 
subaquatie plants, but the whole process is the result of a gra- 
dual extension of the margin. This may readily be examined in 
those lakes of which the interior declivity is gentle, and at 
those points where deltas are formed by the entrance of streams. 
The growth of Arundo phragmites, Scirpus lacustris. Lobelia 
dortmanna, and other plants, here serves to detain much of the 
sand and earth which would otherwise be carried forward into 
the deeper parts of the lake, and thus gradually affords a place 
first to the rushes and grasses which flourish in inundated 
soils, and lastly^ to the various plants which, in succession, 
grow in marshy grounds of different degrees of moisture. 
