VALLEYS OF ELEVATION. 



161 



Granitic masses are found under similar circumstances, in the 

 midst of which rise hillocks of basa'lt or scoriae, which doubtlessly 

 followed the first explosion, as at Monte-Nuovo and the island of 

 St. George. 



14. Calcareous countries are not more exempt from these acci- 

 dents than others; only the crate 'ri form cavities, in place of being 

 nearly circular, are more frequently elliptical, sometimes very 

 much elongated, as seen in the Jura mountains. In general, the 

 length is produced, like cracks, extending to a great distance, and 

 forming along its direction elongated hillocks, in a line with each 

 other, offering here and there more projecting summits. These 

 summits are most frequently rent, and present what are termed 

 dosed valleys, and valleys of elevation (Jig- 264), which are in 

 fact craters of elevation. 



Fig. 264. Plan of a crater of elevation in calcareous countries. 



15. Ruptures of calcareous mountains do not always present the 

 crate' riform uniformity just indicated, but vary much, in this re- 

 spect. One side of the rupture sometimes remains low, while the 

 other is elevated, as represented (Jig. 285). Sometimes the supe- 

 rior beds seem to have retired horizontally, and the inferior strata 

 are arched up between the fractured extremities, as seen (fig. 260). 



Fie?. 265. Fig-. 266. 



Craters of elevation in calcareous formations. 



Often, among the upheaved beds, some are found which are easily 

 disintegrated, and their projection soon tumbles, inducing the fall 

 of solid strata ; from this we have ridges of rock parallel to each 

 other, separated by little valleys, in which the rain-water flows, and 

 they become covered by vegetation ; in this case the general ridg^ 

 of the mountain is as represented (fig. 267). Sometimes the 

 summit only presents a mass of calcareous blocks piled on on the 

 other, but arranged in line, as if the work of a mason. Again, 



14. What are valleys of elevation ? What is the peculiarity of crate'-i. 

 r orrn cavities in calcareous countries ? 



15. Are the crate'riform cavities, in calcareous countries, always uniform 

 in configuration ? 



