CAVERNS. 



255 



The latter is supposed to contain animal matter equal to at least 2500 individuals of the 

 cavern bear ; and allowing an annual mortality of 2 J, it follows that here we have the 

 history of a thousand years ; for probably these animals retired to the solitude of this 

 spot upon the approach of death, as is the well-known 

 custom of many creatures. The bone caverns in our own 

 country and the continent decisively prove that, pre- 

 vious to a great inundation in 

 by-gone time, animals inha- 

 bited these districts, known not 

 to have lived there, 

 from the earliest 'iff 



Section of Gailenreuth Cave. 



records of human 

 history the rhinoceros, 

 elephant, and hya3na, now, and for 

 ages past, exclusively confined to 

 more southern latitudes. 



There is another class of caverns 

 remarkable for the development of 

 irrespirable gas, which often renders 

 the access to them dangerous. They 

 are of two kinds ; those in which the 

 gas is produced by the surrounding 

 rocks, and those in which it pro 



ceeds from the interior of the earth. The first class are principally caves of gypsum. The 

 gypsum is not, however, the cause of the phenomenon, the component parts of which are 

 not susceptible of any decomposition from the air. There is commonly foetid limestone 

 intimately mixed up with it, which forms connected wavy stripes, and even single beds of 

 considerable thickness. This earthy limestone, which is penetrated with bitumen, and 

 often very clayey in its composition, has the property of giving out all its carburetted 

 hydrogen in the air ; and in every case where caves exist in it, its presence, on account of 

 its connection with gas, is offensive and much dreaded. In the limestone caves of the 

 sandstone formations, on the contrary, there commonly prevails a very pure air, possibly 

 because they are filled with mouldering animal remains. The development of irrespirable 

 gases from the interior of the earth, which, penetrating through fissures, collect in caves, 

 is a constant result of volcanic activity. The chemical processes continually going on in 

 volcanic regions must produce the liberation of great quantities of gas, which are con- 

 nected with the world above by these chimneys of the perpetual forge. Caverns of this 

 nature occur therefore only in the neighbourhood of volcanoes, or at points where volcanic 

 processes may be supposed to be going on beneath. The gases so developed are almost 

 entirely the carbonic and sulphuric acids. Among the most important of the grottos 

 which give out carbonic acid, there is the Grotto del Cane at Naples, in the neigh- 

 bourhood of the Lago d'Agnano, near Pozzuoli. It was known to the ancients, and is 

 mentioned by Pliny, who refers to it as one of a class of excavations called, in his 

 time, " Charon's ditches." Its size is very unimportant ; ten feet deep, four feet broad, 

 and nine feet high. The carbonic acid collects itself on the soil in a bed of about six 

 inches deep ; and, on account of its specific gravity, does not mingle with the atmo- 

 spheric air. Its actual height may be clearly ascertained by lighting some candles, 

 which, when they reach its surface, are extinguished at once. Small animals falling 



