VOLCANOES. 711 



300 to 500 feet. la other cases, the central vent continues to eject cinders for a long 

 period ; and the mountain becomes high and steep. 



Where the liquid rock flows from an open vent or pool, like those of Kilauea, the cooled 

 lava has a surface-crust, four to six inches thick, of glassy scoria. The process of boil- 

 ing covers the lavas in the pools with a scum, as it does molasses ; and this scoria is the 

 hardened scum or froth. Below this scoriaceous surface, the lava is solid rock, often 

 containing only a few ragged cellules. 



"When the outflow takes place from fissures, through which the lavas come up with- 

 out having undergone any boiling, the stream is often solid lava throughout, without 

 any scoria ; the surface is hard and compact, but looks ropy, owing to the marks of 

 flowing. 



Whenever the stream of lava stops on its course, it rapidly hardens, over its surface. 

 If it is then made to move again, from another accession of lavas, the hardened crust 

 breaks up like ice on a pond, but makes black and rough cakes and blocks, 100 to 

 10,000 cubic feet in size, which lie piled together over acres or square miles. Such 

 masses are sometimes called clinkers. A large part of the island of Hawaii is covered 

 by the bare lava-streams, — some with twisted, ropy markings over the surface, drawn 

 out as the sluggish liquid flowed along; others, great clinJcer-Jields, horrid exhibitions 

 of utter desolation. 



The streams of lava over the land often rise into great protuberances, many yards 

 across, with oven-shaped cavities within, formed by waters beneath, that were evapor 

 rated by the heat while the flow was in progress. 



In a submarine eruption, or wherever the lavas enter the sea, an upper portion of the 

 outflow, in contact with the water, is shivered to fragments ; if in deep water, the frag- 

 ments are deposited and make a stratum of tufa, sometimes taking a conical form; if at 

 the water's edge, they rise in a shower of water and cinders, and fall around, making a 

 tufa-cone over the opened vent, besides spreading far and wide over the adjoining 

 region ; or they make a permanent boiling basin, which also is the centre of a tufa-cone. 

 This latter kind of tufa-cone has a saucer-shaped crater and the inward and outward 

 slopes of the layers represented in Fig. 1112; while the preceding may fail mostly of 

 the inward slope. 



(b.) Forces causing Eruptions of Lava. — A. Hydrostatic Pressure 

 in the Lava against the Sides of the Mountain. — An increase of 500 

 feet in the depth of the lavas is an increase of 625 lbs. of pressure to 

 the square inch. Such a pressure tends to produce fractures for the 

 escape of the lavas. 



B. Pressure of Vapors. — Vapors rising out of the lavas into any 

 confined space may bring pressure to bear against the sides of the 

 mountain ; and, if suddenly evolved, this effect may cause fracture. 



"Water may come in contact with hot lavas, and enter the spheroidal 

 state (the state in which a drop of water is when it dances about on a 

 red-hot stove) ; and when so, it will suddenly and explosively pass into 

 a state of vapor on cooling. This is supposed to be one cause of ex- 

 plosion in steam-boilers ; and, with the apparatus of a volcanic moun- 

 tain, the results may rend the mountain. In this way, and also through 

 the more quiet evolution of vapors, earthquakes sometimes result. 



C. Lateral Pressure in the Earth's Crust, resulting from Contrac- 

 tion. — This cause has produced subsidences of the crust, in the earth's 

 history ; and such subsidences must have been attended by movements 

 in the underlying liquid rock. This cause must have acted when the 



