NATURE OF ORGANIC REMAINS. 39 
charged with calcareous earth (/ime), and deposit it in con- 
siderable quantity, petrifying springs ; as those of Matlock, 
and other places in Derbyshire. (Wond. p.76.) But in- 
erustations are not petrifactions; stems and branches of 
trees, skulls, bones, shells, &c., are simply invested with a 
calcareous coating or crust, which is generally porous and 
friable, but often crystalline and compact. The inclosed 
bodies are not permeated by the stony matter; if the mass 
be broken, or the incrustation removed, we find the twig, or 
stem, either dry and shrivelled, as in the specimens, figs. 2, 
3, 4, Plate ITI. ; or tubular cavities are left by the decay and 
removal of the vegetable structure, as in fig. 10, Plate III. 
But although incrustations be not petrifactions, natural 
specimens, (not the so-called petrified nests and twigs, in 
which the bad taste of the guardians of the Derbyshire 
springs is embodied, and dispersed all over England,) are 
objects of considerable interest, as illustrative of a process, 
by which important changes are effected in the mineral 
kingdom. Thus springs as clear and sparkling as poet 
ever feigned or sung, may transform beds of loose sand and 
gravel into rock, and porous stone into solid marble, and 
cover extensive tracts of country with layers of concretionary 
_ and crystalline limestone. This process is effected in the 
_ following manner. Most fresh water holds in solution a 
certain proportion of carbonate of lime; and changes of 
temperature, as well as other causes, will occasion this cal- 
careous earth to be in part or wholly precipitated. The 
fur, as it is called, that lines a kettle or boiler which has 
been long in use, affords a familiar illustration of this fact. 
At the temperature of 60° lime is soluble in 700 times its 
weight of water; and if to the solution a small portion of 
carbonic acid be added, a carbonate of lime is formed, which 
is thrown down in an insoluble state. But if the carbonic 
acid be in such quantity as to supersaturate the lime, it is 
again rendered soluble in water: it is thus that carbonate of 
