164 R O 
the effect of art. Thus, the blocks of granite remaining in 
the quarries at Sienna, in Upper Egypt, have all the im¬ 
pressions of the toots with which they were worked during 
the latter period of the Roman empire : and the basaltic rocks 
formed by the extinct volcanoes of Auvergne are so compact, 
as to present, at this day, the appearance of trickling lava, 
suddenly congealed, though the period of their formation 
was prior to the record of authentic history. The causes of 
disintegration are, the vicissitudes of the atmosphere, change 
of temperature, and the absorption and congelation of 
water. The sudden dilation or contraction produced by the 
expansion and congelation of water, is alone sufficient to 
rend the strongest rocks when it enters their fissures. In 
Greenland the rocks are said, from this cause, frequently to 
burst with a noise like thunder. The external causes of 
decomposition are, principally, water and oxygen. Mineral 
substances, containing sulphur or metallic matter capable of 
a higher degree of oxygenation, absorb oxygen from water, 
or the atmosphere. To this cause the decomposition of 
stones, containing pyrites, is to be attributed. Calces of iron, 
moderately oxygenated, are the most general cause of decom¬ 
position ; they act by absorbing a greater portion of oxygen, 
by which they gradually swell, and are disunited from the 
other constituent parts of the stone in whose composition they 
enter. When least oxygenated, their colour is black, or dark 
brown; and, in some instances, when combined with alumine, 
or magnesia, greenish-grey; the alumine, as it becomes more 
oxygenated, turns to a purple-red and orange, and finally a 
pale yellowish-brown ; the magnesia becomes at first blue, 
then purple and red. Iron, in its metallic state, or, at least, 
when but slightly oxygenated, also decomposes water; but if 
exposed to the air, it becomes further oxygenated, and the 
compound into which it enters gradually decays. 
Stones, into whose composition calces of iron, highly oxy¬ 
genated, seem originally to have entered, are said, by Mr. 
Kirwan, to decompose with great difficulty, of which he 
instances red jasper; but perhaps the more perfect vitrification 
of these minerals may be the cause of their durability. 
Calcareous rocks are liable to the decomposing effects of 
water, partly mechanical and partly chemical. Carbonate of 
lime is insoluble in water, except when aided by carbonic 
acid ; but as this exists more or less in almost all water, it 
acts slowly upon limestones, particularly on those of a loose 
texture. Potash and soda enter also into the composition of 
many rocks ; and to the existence of potash in felspar, one of 
the constituents of granite, the decomposition of granite rocks 
may be generally attributed. 
From the combination of some extraneous mineral sub¬ 
stance with rocks, remarkably rapid instances of the decom¬ 
position of rocks are sometimes known to take place. Ac¬ 
cording to Dolomieu, all the houses of Malta are built of a 
fine grained limestone of a loose and soft texture, but which 
hardens by exposure to the air. There is a circumstance 
which hastens its destruction, and reduces it to powder, 
namely, when it is wetted by sea-water; after this it never 
dries, but is covered by a saline efflorescence, and a crust is 
formed some tenths of an inch thick, mixed with common 
salt, nitre, and nitrate of lime. Under this crust the stone 
moulders to dust, the crust fails off, and other crusts are suc¬ 
cessively formed, until the whole stone is destroyed. A single 
drop of sea-water is sufficient to produce the germ of destruc¬ 
tion, which gradually increases and spreads, like a caries, 
through the whole mass of stone ; nor does it stop there, but 
after some time affects all the neighbouring stones in the 
wall. The stones most subject to this decay are those that 
contain most magnesia; those that are finer grained, and of 
a closer texture, resist if. Notwithstanding the speculations of 
Dolomieu and Mr. Kirwan on the cause of this uncommon 
property in the stone of Malta, we conceive that a satisfactory 
explanation is still wanting. 
We have before stated, that water is the principal agent in 
the gradual destruction of rocks.; but sometimes entire moun¬ 
tains are suddenly levelled by subterranean currents, which 
work their passage through fissures and cavities, and silently 
prepare the causes of the most alarming catastrophes in alpine 
C K. 
districts. In proportion to the elevation and abruptness of 
rocks, these catastrophes are more frequent and extended. In 
the cantons of Switzerland numerous instances of this kind 
are on record, and one, w'hich we shall subsequently notice, 
occurred in our own times. 
The town of Fleurs, about a league from Chavennes, con¬ 
taining about 2200 inhabitants, and numerous splendid 
palaces, was suddenly buried under a mountain on the 25th 
of August, 1618. 
On the same evening an inhabitant, who entered the town, 
advised his neighbours to leave the place, saying, he had! seen 
the mountains cleaving, but could obtain no credit; his 
daughter, whom he had persuaded to depart with him, returned 
to lock up some valuables, and was overwhelmed with all the 
other inhabitants in one common ruin. Mr. Coxe says vine¬ 
yards, chesnut trees, and houses, now cover the spot where 
this unfortunate town once stood. 
On the 23d of September, 1714, a great part of the moun¬ 
tain Diableret fell in between two and three o’clock in the 
afternoon, and buried more than 100 huts, and a con¬ 
siderable part of the valley. Those who saw this disaster 
say that it happened in a moment, and at the same time 
whirling clouds of dust arose suddenly, which darkened the 
air like a sudden night. 
In 1751, a mountain fell down, situated near Passy, 
between Salenches and Servoz. Saussure says, the noise was 
so dreadful, and so thick and dark a cloud of dust arose, that 
many persons supposed that the world was at an end. Intel¬ 
ligence was received at Turin, that a terrible volcano had 
broken forth in these mountains; in consequence of which 
the king sent the celebrated naturalist, Vitaliano Donaty, to 
verify the report, who gives the following account of th& 
event. “ I hurried with extreme pleasure to examine so ex¬ 
traordinary a phenomenon. After having travelled four 
days and four nights without halting, I came in front of a 
mountain all covered with smoke, afld from which were in¬ 
cessantly detached, both by day and night, large masses of 
stone with a noise perfectly like that of thunder, or of a large 
battery of cannon, but louder and more terrible. The peasants 
had all retired from the vicinity, and dared not to look at the 
ruin, until at the distance of two miles, or even farther. All 
the neighbouring hills were covered with a dust much 
resembling ashes. ‘All said they had seen at intervals a smoke, 
which was red during the day, and accompanied with flames 
at night. I attentively examined the smoke, but neither per¬ 
ceived flames nor any smell of sulphur. Nor did the rivulets 
I examined with care, present the least appearance of sul¬ 
phuric matter. Thus persuaded, I entered the smoke, and 
though quite alone, went to the brink of the abyss, where I 
saw a large rock darted down, and observed that the smoke 
was only dust raised by the fall of the rocks, the cause of 
which 1 soon after sought for and discovered.—A great part 
of the mountain situated above that which had fallen was 
composed of earth and stones, not disposed in beds, but 
confusedly heaped together. I thus perceived that the moun¬ 
tain had been subject to similar falls, which had left the 
large rock that fell this year without a support, and with a 
considerable projection. This rock was composed of hori¬ 
zontal beds, of which the lower were of slate, or rather of 
fragile schistose stone of little consistency, while the two beds 
beneath these were of marble like that of Porto Venere, but 
full of fissures, which crossed the beds. The fifth bed was 
wholly composed of slate in vertical plates, entirely dis¬ 
united ; this bed formed all the upper part of the fallen 
mountain. Upon the same level summit were three lakes, 
the waters of which penetrated constantly through the fissures 
of the beds of rock, and decomposed their supports. The snow, 
which had fallen in Savoy in greater abundance than had ever 
been seen in the memory of man, increased the effect, and 
the united waters occassioned the fall of three million cubic 
fathoms of rock, a mass sufficient to form a large mountain.” 
On the 2d of September, 1806, at five in the evening, the 
Knippenhoui rock, which formed the summit of Mount 
Rosenberg, was on a sudden detached from its situation; and 
at the same time part of the mountain, about 280 feet thick 
on 
