Loudon free press. (Loudon, Tenn.) 1852-1855, 1854-04-04
ON A CHEMICAL CAUSE OF CHANGE
IN THE COMPOSITION OF ROCKS.

The following is an abstract of paper read
before the British Association, by Prof John
son. The first example Of chemically alter
ed rock adduced by the Professor, was the rot
ten.stone of Derbyshire light and porous sub
stance used chiefly for polishing metals, and
stated in Philips "Mneralogy" to be composed
of silica, alumina, and carbon. 1t is obtained
from ridge covered with "drift" 10 or 20 feet
thick, consisting of brown clay, with names of
black marble ebert and rotten-stone. The rot
ten-stone is SO soft whilst in the soil that the
spade goes through it readily, but it hardens on
exposure; the holes from which it is dug are
sometimes only feet deep, at others from to
8 feet. On examining a series of specimens,
Prof Johnston found that whilst some were
homogeneous, Others had nucleus of black
marble; he then treated specimens of the black
marble with acid, aud found that Oh the re-
moval of the carbonate of lime there remained
from Is to 20 per cent. of a slicious substance
perfectly like the natural rotten-stone. He con-
cluded that there existed in the soil some acid
which penetrated it and dissolved out the calca-
reous matter of the rocks below. The agent IN
this case might be the carbonic acid OF the air,
brought down by rain; but there were instances
not capable ofeX olanation by this agency alone,
and attributable to other acids, which are pro-
duced under certaiBs conditions and exercise
much wider influence. The bottoms of peat
boys present very strong evidence of the action
of acids, the stone and clay are beached and
corroded only silicious and colorless materials
being left. The source of the acid is here the
same as in the former instance; the vegetable
matter growing on the surface produced in its
decay acid substances which exert a chemical
action on the subsoil, and escape by subterra-
IL 81, -,
solved in their progress. Another instance was
afforded BY the mineral pigotite, formed in the
caves of Cornwall by water dripping from the
roof; this water contains peculiar organic
acid, derived from the soil Of the moors, which
Gissolves the alumna of the granite and com
bines with it. The organic acids are very nu.
merous and different in composition, but, agree
in producing chemical action upon rocks
They are produced over the entire surface Of
the earth, especially over uncultivated tracts,
ana are the means provided by nature to dis
solve the mineral food of plants; they are also
amongst the chief causes of the exhaustion of
soils. The author then alluded to Prof Ways
examination of some of The green-sand strata
of Surrey, known as fire stone,-a light and
porous rock, containing silica in soluble state.
1t was well known that common sand stone,
quartz, or rock crystal we're not acted upon by
potash or soda at ordinary temperatures; but of
the fre-sone 30 per cent, and sometimes so or
20 per cent. may be dissolved. In all such
cases the silica must have been originally in
state of chemical combination with ime alumni
ma, or something else, which has been subse-
quently removed. The silica in the rotten-stone
was soluble, but he had never met with instan-
ces of black marble in bedded state converted
into rotten stone. He believed, however, that
similar cause, operating over wide area, and
during long period, had produced the altered
condition of the firestone. Plof Johnston then
alluded to the nodulcs of phosphate of lime in
the green-sand and crag, and suggested that the
phosphorus had been derived from animal re-
mains in higher strata, dissolved out by acids
and re deposited at lower level. The last ex-
ample was the fireclay of the coal measures,
stratum almost universally found beneath beds
of coal 1t differs from the other clay both in
color and composition, being whittier and CON
tanning less of those substances which acid bod
les could dissolve, viz, the earthly basis, which
would render the clay fusible in fire; the con-
dition of the fire clay might be accounted for
by the action of acids developed during the pro-
duction of the vegetable matter now forming
COAL Scientific American.