Freshwater Limestone in Forfarshire, Sgc. 81 



ments by which in the first instance that eaith is secreted from the water; and 

 from these plants, through the medium of food, it passes into the testacea. 

 Accordingly, unless there are springs, a lake does not produce marl in any 

 quantity, even though the lake be situated in one of the sandstone districts 

 most favourable to its production. The growth of testacea, in such cases, is 

 necessarily very limited, and their exuviae cease to accumulate ; because it is 

 only from the decomposition of the exuviee of a former race that the water- 

 plants can obtain the lime which is requisite for their own support, and ulti- 

 mately for the growth of a new race of testacea. In conformity with these 

 views, we find that it is always near the springs that the marl is in the great- 

 est quantity; because it is near to them that the testacea delight to congregate, 

 partly owing to the greater clearness of the water, but principally to its uni- 

 form temperature, which occasions the water-plants to vegetate there in the 

 very depth of winter. For the marl to be abundant, the springs must be co- 

 pious ; for it to be pure, the lake must be clear and free from muddy sedi- 

 ment ; and must be fed therefore not by rivers, but by springs, as is the case 

 with the Loch of Bakie. 



On the Origin of the Tufaceous Limestone or Rock-marl. 



The origin of the limestone or rock-marl appears to admit of only two rea- 

 sonable explanations. 1st, It may be regarded as a tufaceous deposit from 

 the springs in the bottom of the lake, which quitting their sources surcharged 

 with carbonate of 'lime, on mixing with the water of the lake deposited 

 that surcharge. As the tufas deposited in the open air from petrifying springs 

 include terrestrial and aquatic plants and shells, so would tufa, formed, as upon 

 our hypothesis, underwater, be found to envelope the native plants and testa- 

 cea of the lake. But however natural this hypothesis may appear, the parti- 

 cular circumstances of the case do not warrant our adopting it ; for the springs, 

 instead of being suixharged with lime, contain only a small proportion of that 

 earth ; and consequently neither do they now, that the lake is drained, form a 

 calcareous precipitate around their sources ; nor can they have done so for- 

 merly at the bottom of the lake, before it was drained; unless (which is highly 

 improbable) at some former period they contained lime in solution more co- 

 piously than at present. It is unnecessary therefore to dwell upon this hypo- 

 thesis. We ought the more readily to discard it, because, although the springs 

 in Forfarshire and the adjoining part of Perthshire contain an abundance of 

 carbonic acid, they have not yet any where, except in connexion with shell- 

 marl, been observed to deposit tufa. 



2dly, The limestone or rock-marl may be considered as formed by the ac- 



VOL. II. — SECOND SERIES. M 



