48 
Br MacCulloch on Peat, 
Although the submerged wood of peat-bogs thus continues 
often unchanged for a long period, in other cases it undergoes a 
chemical change, without loss of texture, arid acquires the pe- 
culiar properties of peat. It will hereafter be shewn, that a 
longer continuance of the same action of water produces an in- 
cipient bituminization, and that time alone is probably required 
to convert such wood into lignite, or into a substance resembling 
brown coal, surturbrand, or even jet ; products, of which the pe- 
culiarities are in some measure owing to collateral circumstances, 
which it is unnecessary at present to consider. 
It is scarcely necessary, in concluding this subject, to remark, 
that forest-peat must vary materially, according to the pro- 
portions in which wood and the ordinary marsh plants enter into 
its composition, independently of the numerous other circum- 
stances by which the characters of every variety of this substance 
are modified. 
The trees which in Scotland enter into the formation of forest- 
peat, are chiefly oak ^ fir, alder, and birch. To these may be ad- 
ded, as more rare, the ash, the roan, the hazel, and difierent spe- 
cies of willow. To enumerate the plants which grow under their 
shade, or occupy the soil on which they have fallen, would, 
with a few trifling additions, be to repeat much of the cata- 
logues which have already been given. 
It has scarcely been noticed by our writers on this subject, 
that peat is also occasionally formed on sea-shores, by the death 
and renewal, in salt-marshes, or in other analogous spots, of 
those plants which affect maritime situations. Such peat may, 
however, be found on the shores of the maritime Highlands, and 
in other analogous places, and it often forms strata alternating 
with sand, or with the muddy deposits of rivers. It is not un- 
common, in those places where the estuary of a considerable 
river is a sea-loct, and where, by the gradual extension of the 
shallow shores, from the same causes which operate in shoaling 
inland lakes, the sea becomes gradually excluded. West Loch 
Tarbet and Isla, in this country, present, among other places, 
very accessible and conspicuous examples of this process, in its se- 
veral stages. In the former spot, it may be seen in a state of great ' 
activity, and a considerable accession of land is thus gradually 
taking place. In Isla, it has long since terminated in the flat 
3 
