103 



VOLCANIC PHENOMENACRATERS. 



us in great craters of elevation by the crevices or cracks which 

 extend from the margin of the escarpment to the external base of 

 the mountain, forming what are named burancos in the Canary 

 islands, where the) ire so remarkable. One of these barancos 

 (or ravines) much deeper than the others, extends from the foot ot 

 the mountain to the bottom of the crater, as is shown in the follow- 

 ing view (Jig. 189). This last character is seen almost always in 



Fig. li s '9. View of the Island of Falma. 



tne different localities produced by similar events, as well as m 

 most islands which have been upheaved in our times in the midst 

 of the ocean ; frequently there are many valleys of the same kind. 



Remaiks on the formation of craters. We have mentioned explosion as 

 determining, definitely, the formation of the crate'riforrn cavity at the sum- 

 rnii of the upheaved mass ; however, it is not probable that this circum- 

 stance, which is applicable to Monte-Nuovo, the island of St. George, &c., 

 is constantly seen in all cases ; it seems to be even totally inadmissible in 

 certain craters of vast extent known to exist in a number of places. But 

 this explosion is not even necessary. In fact it is easy to conceive that 



after a fracture, as in Jig. 190, 

 which is a correlative result of ele- 

 vation, it may happen that all the 

 erect, column-like masses, and all 

 the elongated points between the 

 rents, might be tumbled down at the 

 same moment, or by a subsequent 

 action. Hence results an open cavi- 

 ty (Jig. 191), the margin of which 

 is formed by all the debris, and the 

 depth is in proportion to the sum of 

 the voids or spaces formed by the 

 fractures. On the other hand, it is 

 clear that elevation is produced by 

 gome matter, liquid or gaseous, 

 which pushes the crust of the earth 

 and forces it to swell upwards ; now, 

 if it happen that this matter should 

 find exit at some other point, or re. 

 tire again into the bowels of the 

 earth, the upheaved part being left 

 without support may sink into the 

 abyss left beneath it, and conse- 

 quently cause an immense vacuity 

 in the midst of the gibbosity or 

 Fig. 191. hillock, then merely forming amass 



16. How are craters of elevation distinguished when the upheaved mat- 

 ters are not divided into beds ? 



