Dr MacCulioch on Peat 
41 
The quality of peat varies principally according to the diffe- 
rent situations in which it is formed, as these differ in the de- 
gree of drainage and of temperature, and in the nature of the 
vegetables which they produce. Hence agriculturists have dis- 
tinguished several varieties, which have, however, been sometimes 
also named from differences of texture, at others from differences 
of situation. The former distinctions have been founded either 
on the more or less perfected state of the process, or on acciden- 
tal circumstances, which will hereafter be apparent. The whole 
of these varieties may be included in the following divisions : 
namely, Mountain, Marsh, Forest, Lake, and Marine, Peat. To 
these ought to be added transported peat, which, although of 
very partial occurrence in this country, appears to abound in 
Holland, and in other parts of the continent of Europe. 
These several essential varieties occasionally present many in- 
ferior distinctions of quality and aspect, arising either from the 
nature of the plants which have contributed to their formation ; 
from the varying extent to which the process has been carried ; 
from intermixture of character, dependent on the variable state 
of drainage or other causes, or from the alternation or admix- 
ture of transported earth or of shells. In some instances, even 
in situations very different, they approach so nearly in character 
as to be undistinguishable ; and this happens more particularly 
in the peat of lakes and in that of marshes, and in those cases 
where the process is most complete. 
In general, peat forms a single superficial stratum, lying 
either on the solid rock, as in the islands of North and South 
Uist, Lewis, and the remainder of that chain, or on the allu- 
vial soil which, in other places, is found above the fundamen- 
tal rock. In other cases it is found to alternate with sand and 
gravel, with clay, or with shell marl ; while, in a few instances, 
it is found at considerable depths, buried under a thick bed of 
clay, or of other alluvial soil. The alternations of sand and 
gravel occur either on sea-shores, at the estuaries of rivers, or 
their termination in lakes, or in other situations where large 
quantities of these materials are carried down by rivers, so as 
for a time to cover the plain, and destroy the process of vegeta- 
tion. In those cases where large deltas are formed or bays are 
filled up, the deposition of earth c'ontinues to proceed, till the 
