143 



there would remain to examine, among the se- 

 condary rocks, a third formation, that of the 

 argillaceous sandstone*, newer than! the brine- 

 spring formations: but this rock, composed of 

 small grains of quartz cemented by clay, sel- 

 dom contains caverns ; and when it does, they 

 are not extensive. Progressively narrowing to- 

 ward their extremity^, their walls are covered 

 with a brown ochre. 



We have just seen, that the form of grottoes 

 depends partly on the nature of the rocks in 

 which they are found ; but this form, modified 

 by exterior agents, often varies also in the same 

 formation. The configuration of caverns, like 

 the outline of mountains, the sinuosity of val- 

 Iies, and so many other phenomena, present at 

 first sight only irregularity and confusion. The 

 appearance of order is resumed, when we can 

 extend our observations over a vast space of 

 ground, which has undergone violent, but pe- 

 riodical and uniform revolutions. From what 

 I have seen in the mountains of Europe, and in 

 the Cordilleras of America, caverns may be 

 divided, according to their interior structure, 

 into three classes. Some have the form of large 

 clefts, or crevices, like veins not filled with ore i; 



'* Sandstone 'of Weisenfels and of Nebra, oolite sand- 

 stone, bunte sandstein. 



t Such are the Heuscheune in Silesia j the Diebskeller, 

 and the Kuhstall, in Saxony. 



