22G COSMOS. 



craters of elevation ; and, lastly, the elevation of a permanent 

 volcano in the crater of elevation, or among the debris of its 

 earlier formation. At different periods, and in different de- 

 grees of activity and force, the permanent volcanoes emit 

 steam, acids, luminous scoria3, or, when the resistance can he 

 overcome, narrow, band-like streams of molten earths. Elas- 

 tic vapors sometimes elevate either separate portions of the 

 earth's crust into dome-shaped unopened masses of feldspathic 

 trachyte and dolerite (as in Puy de Dome and Chimborazo), 

 in consequence of some great or local manifestation offeree in 

 the interior of our planet, or the upheaved strata are broken 

 through and curved in such a manner as to form a steep rocky 

 ledge on the opposite inner side, which then constitutes the in- 

 closure of a crater of elevation. If this rocky ledge has been 

 uplifted from the bottom of the sea, which is by no means al- 

 ways the case, it determines the whole physiognomy and form 

 of the island. In this manner has arisen the circular form of 

 Palma, which has been described with such admirable accu- 

 racy by Leopold von Buch, and that of Nisyros,^ in the ^gean 

 Sea. Sometimes half of the annular ledge has been destroy- 

 ed, and in the bay formed by the encroachment of the sea cor- 

 allines have built their cellular habitations. Even on conti- 

 nents craters of elevation are often filled wdth water, and em- 

 bellish in a peculiar manner the character of the landscape. 

 Their origin is not connected wdth any determined species of 

 rock : they break out in basalt, trachyte, leucitic porphyry 

 (somma), or in doleritic mixtures of augite and labradorite ; 

 and hence arise the diflerent nature and external conformation 

 of these inclosures of craters. No phenomena of eruptions are 

 manifested in such craters, as they open no permanent channel 

 of communication with the interior, and it is but seldom that 

 we meet with traces of volcanic activity either in the neigh- 

 borhood or in the interior of these craters. The force which 

 was able to produce so important an action must have been 

 long accumulating in the interior before it could overpower the 

 resistance of the mass pressing upon it ; it sometimes, for in- 

 Btance, on the origin of new islands, will raise granular rocks 

 and conglomerated masses (strata of tufa filled with marine 

 plants) above the surface of the sea. The compressed vapors 

 escape through the crater of elevation, but a large mass soon 

 'iiUs back and closes the opening, which had been only formed 

 by thsse manifestations of force. No volcano can, therefore, 



* See the interesting little map of the island of Nisyros, in Ross's 

 lieisen auj den OHechischen Inseln, bd. ii., 1843, s. 69. 



