GEOLOGY. 323 



and of all the districts where chalk strata occur ; 

 these beds consisting principally of fragments of the 

 common black flint altered by exposure to the air, 

 and very uniformly worn; but still the properties 

 sufficiently correspond to establish their identity. 



Sandstone and Limestone Hills. 



The sandstone and limestone hills in Derbyshire 

 and North Wales may easily be distinguished at a 

 distance in Summer, by the different tints of the 

 vegetation. The grass on the sandstone hills usually 

 appears brown and burned up, while that on the 

 limestone hills is green and flourishing.* 



Volcanoes. 



Werner thinks there are 193 volcanoes known to 

 be in activity on the face of the earth, besides a 

 much greater number extinguished, or dormant, or 

 under the sea. 



The simultaneous eruptions of volcanoes, and 

 occurrence of earthquakes, in places distant from 

 each other, might be adduced to prove subterranean 

 communication between the volcanoes. 



The enormous volcanic cones of Cotopaxi, Pe- 

 chinga, Tanguragua, &c. in South America, never 

 throw out lava, but frequently ashes, scoriae, and 

 pumice, and sometimes immense quantities of muddy 

 water. 



It would appear that, in America, all elevations 

 higher than Montblanc are entirely composed of 

 volcanic matter. 



* Carbonate of lime (common limestone) is easily recognized 

 by its softness, and by its effervescence with dilute sulphuric 

 acid. If exposed to heat under pressure, so as to prevent the 

 escape of gazeous matter, it fuses and retains its carbonic acid. 

 None of the other carbonates exist in such large mineral masses. 

 Y 2 



