240 PHYSICAL GEOGRAPHY. 



immense, and calculated to produce vacuities in which the largest mountains would have 

 ample space. It has been estimated that Etna in one of its last most important erup- 

 tions, that of the year 1769, threw out a mass of lava equal in volume to a cone 5820 

 feet in height, and 11,640 feet in breadth, or nearly four times larger than Vesuvius. 

 Fourteen such eruptions would produce a mass equal to Mont Blanc, reckoning from the 

 level of the sea, and twenty-six such large eruptions have occurred since the twelfth 

 century. In .the year 1783, when the earthquake of Calabria occurred, the Skaptar 

 volcano in Iceland poured forth a stream of lava fifty miles long, between twelve and 

 fifteen broad, and from one to six hundred feet in thickness, which must have been 

 equal to six times the mass of Mont Blanc, and two and a half times that of Chimborago. 

 Hence, it is a common event, in countries subject to great volcanic activity, for portions 

 of the surface to fall in, the subsidence frequently becoming the bed of a lake. A part 

 of the forest of Aripas in the Caraccas thus subsided in the year 1790; a lake was 

 formed nearly half a mile in diameter, and from eighty to a hundred yards in depth, 

 and for several months after the trees of the forest remained green under the water. In 

 the same year, in Sicily, at Santa Maria de Nisremi, a portion of the country three 

 Italian miles in circumference sank thirty feet deep. Occurrences of the same kind 

 appear to take place in the depth of the sea, the falling in of its bed being indicated on 

 the surface of the waters by their sudden retreat and violent agitation on their return. 

 A remarkable example of this phenomenon took place at Marseilles, on June 28, 1812, 

 when the water in the harbour suddenly sank, then rushed out with great rapidity, and 

 returned with equal violence; a movement which was repeated several times, till the 

 equilibrium was restored, occasioning considerable damage to the shipping. Instances 

 of similar events are innumerable, which serve to prove the existence of cavities, both 

 in the interior of the exposed crust of the earth, and these parts of it over which the 

 ocean rolls. 



To Humboldt we are indebted for a large amount of information respecting the cavities 

 which appear upon the surface, the chief differences of their form, the beds in which they 

 are found, and the causes which may have originated them. In the primary rocks, 

 caverns are relatively fewer than in the later deposits, while the oldest masses of the 

 granite and gneiss formations are particularly destitute of them. The principal are wide 

 fissures, sometimes of unknown depth, and those hollow passages which occur in 

 Switzerland and Dauphine, called crystal caves, owing to their walls being richly 

 furnished with pillars of rock crystal. Similar vacuities occur in the gneiss of the 

 Pine mountain in the neighbourhood of Wiesenthal, but they are not important. 

 In Sweden and Norway, the granite presents fissures and caves of extraordinary 

 extent, and perfectly unexplored, hitherto ; such as the cave of Marienstadt. the end 

 of which is not known, and the enormous deep hole at Frederick stall, where a 

 stone thrown in only gives the echo of its fall in a minute and a half or two minutes ; an 

 observation which, if well founded, would give, on the calculation of Perrit, a precipitous 

 depth of 59,049 feet, the highest estimate, or 39866 feet, the least ; that is, from twice to 

 three times the height of Chimboraco. It is the primitive limestone that supplies the most 

 numerous examples of caves and grottos in the primary rocks ; and if these yield in point 

 of size to the later limestone formations, this arises from the inferior extent of the 

 primitive limestone, rather than from its incapacity to form caves. In the transition 

 mountains, and those of stratified structure, it is still the limestone in which the more 

 extensive caves are found, of which those of the Hartz, the splendid caverns of 

 Derbyshire, and those of the Carpathians, are well known. Caverns most frequently 

 occur in the mountains of stratified limestone; and among these, one of the* most modern 

 formations, the Jura limestone is particularly distinguished, and was therefore termed 



