ROCK. 



rocks prefent the fame unvarying outline for ages, and 

 preferve their angular (harpnefs, which is either natural, 

 or, in fome inftances, the efted of art. Thus, the blocks 

 of granite remaining in the quarries at Sienna, in Up- 

 per Egypt, have all the imprefiions of the tools with 

 which they were worked during the latter period of 

 the Roman empire; and the bafaltic rocks formed by the 

 extina volcanoes of Auvergne are fo compaft, as to prefent, 

 at this day, the appearance of trickling lava fuddenly con- 

 cealed, though the period of their formation was prior to 

 the record of authentic hiftory. The caufes of difintegration 

 are, the viciflitudes of the atmofphere, change of tempe- 

 rature, and the abforption and congelation of water. The 

 fudden dilation or contradion produced by the expanfion 

 and congelation of water, is alone fufficient to rend the 

 ftrongefTrocks when it enters their figures. In Greenland 

 the rocks are faid, from this caufe, frequently to burit with 

 a noife like thunder. The external caufes of decompofition 

 are, principally, water and oxygen. Mineral fubitances, 

 containing fulphur or metallic matter capable of a higher 

 degree of oxygenation, abforb oxygen from water, or the 

 atmofphere. To this caufe the decompofition of ftones, con- 

 taining pyrites, is to be attributed. Calces of iron, mode- 

 rately oxygenated, are the moft general caufe of decompo- 

 fition ; they aft by abforbing a greater portion of oxygen, 

 by which they gradually fwell, and are difumted from the 

 other conlHtuent parts of the itone in whofe compofition 

 they enter. When leaft oxygenated, their colour is black, 

 or dark brown ; and in fome inftances, when combined with 

 alumine, or magnefia, greenifh-grey ; the alumine, as it be- 

 comes more oxygenated, turns to a purple-red and orange, 

 and finally a pale ycllowifh-brown ; the magnefia becomes 

 at firft blue, then purple and red. Iron, in its metallic ftate, 

 or, at leaft, when but (lightly oxygenated, alfo decompofes 

 water ; but if expofed to the air, it becomes further oxyge- 

 nated,' and the compound into which it enters gradually 

 withers, as Dr. Higgins obferved, in imitating pozzolana. 

 Higgins, on Cements, 124. 



Stones, into whofe compofition calces of iron, highly 

 oxygenated, feem originally to have entered, are faid, by- 

 Mr. Kirwan, to decompofe with great difficulty, of which 

 he inftances red jafper ; but perhaps the mors perfed vitrifi- 

 cation of thefe minerals may be the caufe of their durability. 

 Calcareous rocks are liable to the decompofing effefts of 

 water, partly mechanical and partly chemical. Carbonate of 

 lime is infoluble in water, except when aided by carbonic 

 acid ; but as this exifts more or lefs in almoft all water, it 

 afts flowly upon lime-ftones, particularly on thofe of a loofe 

 texture. Potafh and foda enter alfo into the compofition 

 of many rocks ; and to the exiftence of potafli in felfpar, 

 one of the conftituents of granite, the decompofition of 

 granite rocks may be generally attributed. 



From the combination of fome extraneous mineral fub- 

 ftance with rocks, remarkably rapid inftances of the decom- 

 pofition of rocks are fometimes known to take place. Ac- 

 cording to Dolomieu, all the houfes of Malta are built of a 

 fine grained lime-ftone of a loofe and foft texture, but which 

 hardens by expofure to the air. There is a circumftance 

 which hallens its deftruftion and reduces it to powder, 

 namely, when it is wetted by fea-water ; after this it never 

 dries, but is covered by a faline efflorefcence, and a cruft 

 is formed fome tenths of an inch thick, mixed with com- 

 mon fait, nitre, and nitrate of lime. Under this cruft the 

 ftone moulders to dull, the cruft falls off, and other crults 

 are fucceffively formed, until the whole ftone is deftroyed. 

 A fingle drop of fea-water is fufficient to produce the 



germ of definition, which gradually increafes and fpreads, 

 like a caries, through the whole mafs of ftone ; nor does it 

 flop there, but after fome time affefts all the neighbouring 

 ftones in the wall. The ftones moft fubjeft to this decay 

 are thofe that contain moft magnefia ; thofe that are finer 

 grained, and of a clofer texture, refill it. Notwithftanding 

 the fpeculations of Dolomieu and Mr. Kirwan on the caufe 

 of this uncommon property in the ftone of Malta, we con- 

 ceive that a fatisfaftory explanation is Hill wanting. 



We have before flated, that water is the principal agent 

 in the gradual deftruction of rocks, but fometimes entire 

 mountains are fuddenly levelled by fubterranean currents, 

 which work their paflage through fifi'ures and cavities, and 

 filently prepare the caules of the moft alarming cataftrophes 

 in alpine diilrifts. In proportion to the elevation and ab- 

 ruptnefs of rocks, thefe cataftrophes are more frequent and 

 extended. In the cantons of Switzerland numerous in- 

 ftances of this kind are on record, and one, which we fhall 

 fubfequently notice, occurred in our own times. 



The town of Pleurs, about a league from Chavennes, con- 

 taining about 2200 inhabitants, and numerous fplendid 

 palaces, was fuddenly buried under a mountain on the 

 25th of Auguft, 1618. 



On the fame evening an inhabitant, who entered the 

 town, advifed his neighbours to leave the place, faying, he 

 had feen the mountains cleaving, but could obtain no credit ; 

 his daughter, whom he had perfuaded to depart with him, 

 returned to lock up fome valuables, and was overwhelmed 

 with all the other inhabitants in one common ruin. Mr. 

 Coxe fays vineyards, chefnut trees, and houfes, now cover 

 the fpot where this unfortunate town once flood. 



On the 23d of September, 17 14, a great part of the 

 mountain Diableret fell in between two and three o'clock 

 in the afternoon, and buried more than 100 huts, and a con- 

 fiderable part of the valley. Thofe who faw this difafter 

 fay that it happened in a moment, and at the fame time 

 whirling clouds of dull arofe fuddenly, which darkened the 

 air like a fudden night. 



In 1 75 1, a mountain fell down, fituated near Pafl'y, be- 

 tween Salenches and Servoz. Saufiure fays, the noife was 

 fo dreadful, and fo thick and dark a cloud of dull arofe, 

 that many perfons fuppofed that the world was at an end. 

 Intelligence was received at Turin, that a terrible volcano 

 had broken forth in thefe mountains : in confequence of 

 which the king fent the celebrated naturaliit, Vitaliano 

 Donati, to verify the report, who gives the following ac- 

 count of the event. " I hurried with extreme pleafure to 

 examine fo extraordinary a phenomenon. After having 

 travelled four days and four nights without halting, I came 

 in front of a mountain all covered with fmoke, and from 

 which were inceflantly detached, both by day and night, 

 large mafles of ftone with a noife perfectly like that of 

 thunder, or of a large battery of cannon, but louder and 

 more terrible. The peafants had all retired from the vici- 

 nity, and did not dare to look at the ruin, but at the dif- 

 tance of two miles, or even farther. All the neighbouring 

 hills were covered with a dull much refembling afhes. All 

 faid they had feen at intervals a fmoke, which was red 

 during the day, and accompanied with flames at night. I 

 attentively examined the fmoke, but neither perceived flames 

 nor any fmell of fulphur. Nor did the rivulets I examined 

 with care, prefent the leaft appearance of fulphuric matter. 

 Thus perfuaded, I entered the fmoke, and though quite 

 alone, went to the brink of the abyfs, where I faw a large 

 rock darted down, and obferved that the fmoke was only 

 dull railed by the fall of the rocks, the caufe of which I 



foon 



