252 THE OLD RED SANDSTONE. 
the depositions ; and, as a necessary consequence, frequent 
shiftings of currents. The ocean, too, seems to have les- 
sened its general depth, and the bottom to have lain more ex- 
posed to the influence of the waves. And hence one cause, 
added to the porous nature of the matrix, and the diffused 
oxide, of the detached, and, if I may so express myself, 
churchyard character of its organisms. 
Above the blended conglomerates and sandstones of this 
band a deposition of lime took place. Thermal springs, 
charged with calcareous matter slightly mixed with silex, seem 
to have abounded, during the period which it represents, over 
widely-extended areas ; and hence, probably, its origin. An 
increase of heat from beneath, through some new activity 
imparted to the Plutonic agencies, would be of itself sufficient 
to account for the formation. Ihave resided in a district in 
which almost every spring was charged with calcareous earth ; 
but in cisterns or draw-wells, or the utensils in which the 
housewife stored up for use the water which these supplied, 
no deposition took place. With boilers and tea-kettles, how- 
ever, the case was different. The agency of heat was 
brought to operate upon these ; and their sides and bottoms 
were covered, in consequence, with a thick crust of lime. 
Now, we have but to apply the simple principles on which 
such phenomena occur, to account for widely-spread_ precipi- 
tates of the same earth by either springs or seas, which at 
a lower temperature would have been active in the forma- 
tion of mechanical deposits alone. The temperature sunk 
gradually to its former state; the purely chemical deposit 
ceased ; the waters became populous as before with animals 
of the same character and appearance as those of the up- 
per conglomerate; and layer after layer of yellow sand- 
stone, to the depth of several hundred feet, were formed as 
