45 
Dr MacCiilloch an Peat 
Thus a stratum of peat is gradually formed from the original 
margin inwards, covering, by its extension, the bottom of the 
lake, till, in certain cases, the water is entirely excluded, and 
the whole becomes a peat-bog, or, as may happen in other cir- 
cumstances, a meadow. Under such a stratum of peat, the 
shells which have been deposited in the course of years, are at 
length consolidated into the marl beds, formerly noticed, which 
are so frequent under the flat peat-bogs of the Highlands. 
These are often of such an extent and thickness, as to form 
valuable quarries, if this name may be adopted, for the purposes 
of agriculture ; the calcareous manure which they afford, being of 
the best quality. Many mossy lands have thus acquired a 
value which was little foreseen, and by which the proprietors or 
purchasers have very unexpectedly profited. The discovery of 
these marl -pits has generally been the result of accident, or of 
the excavations used for procuring fuel ; very few of the far- 
mers or proprietors being aware of the true nature and origin 
of these deposits. A more general diffusion of that knowledge 
among them, would lead to the discovery of marl in many parts 
of the Highlands, where it is as yet unknown, and where cal- 
careous manure is, from its rarity and distance, extremely 
valuable. As the shell-fish, which contribute to its formation, 
exist even in very small collections of water, it may often be 
found in the mountain peat-mosses of very limited extent, as 
well as in the larger, which have resulted from the partial or 
total obliteration of considerable lakes. It may be sought for 
with a prospect of success, in all those mosses which are bounded 
by rocky or elevated margins, or which occupy basin-shaped 
cavities. Some of these still give passage to the river which 
once flowed into the original lake, and which has, by its depo- 
sits of earth, contributed to fill the cavity ; while others, which 
give passage to no river, have been produced by the gradual in- 
crease of vegetation from the margin in the mode just described. 
Experience in the forms and disposition of existing lakes and 
pools, will easily guide the eye in the selection of those spots 
where marly deposits may occasionally be expected. It must, 
however, be added, that marl is sometimes found on declivi- 
ties, mixed with, and buried under mountain-peat. But these 
very singular deposits are rare, and the produce of terrestrial 
