VOLCANOES. 745 



feet above the sea-level, and previous to those of Kilauea, 3,400 feet ; 

 that the filling of the craters to the high-level mark had in each case 

 gone on quietly ; that the discharge of the lavas, at the eruptions 

 (among the grandest on record) has usually begun and ended in a 

 quiet way. 



The clouds which constantly rise when the craters are in full action, 

 and often drop rain over the region around, are evidence that water is 

 present within the lavas in immense quantities. Sulphurous acid con- 

 stitutes a very small portion of the escaping vapors. This water is 



Fig. 1122. 

 m 



Section of Mount Loa. 



not salt-water; for chlorides are rarely met with among the prod- 

 ucts due to the vapors. It must come either from deep depths in the 

 earth's crust, as urged by Scrope (first in 1825), or from the rains or 

 superficial waters. Whether the former is one source or not, the lat- 

 ter must influence greatly the action. Rains fall abundantly over the 

 windward (or eastern and southeastern) sides of the island, and make 

 few surface-streams only because the rocks are so extensively fissured 

 and cavernous. Of the water that becomes subterranean, part follows 

 the sloping beds seaward ; but another part must reach the region of 

 the fires, arid after being changed to vapor by the hot rocks about the 

 conduit, pass into the liquid lavas, being forced to this because there 

 are no sufficient passages for the escape of such volumes of vapor or 

 gas. Ehrenberg's observation that the cinders of volcanoes all over 

 the globe contain large numbers of fresh-water infusoria {Diatoms, 

 etc.), supports the view that fresh waters are commonly the working 

 agent. 



The moisture absorbed, or present in the lavas from any subterra- 

 nean source, has a temperature above 2,200° F. (for, if this degree ex- 

 ist at surface, a higher must exist below), and is under a pressure of 85 

 to 95 atmospheres for every 1,000 feet of depth. The superheated 

 steam taken in is forced upward by the heavy lava ; and as it rises, it 

 expands with the diminution of pressure, enlarging the mass of liquid 

 material and thereby increasing its height in the conduit. The steam 

 thus makes its way to the surface. At the surface, the cohesion of the 

 lavas, increased by incipient cooling, detains the bubbles of steam 

 until they are so large and the crust so thin that the steam is able to 

 overcome the resistance ; they then burst, producing the jet of frag- 

 ments, which are liquid lava as they rise, and still liquid enough at 

 Kilauea on falling to adhere firmly to the solid rim of the lava-vent. 



