694 DYNAMICAL GEOLOGY. 



The area of greatest heat was near the northeast side of the lake, 

 and the stream seemed to flow to the southwest. 



3. Eruptions. — (a.) General facts. — The rising of the lavas within 

 the crater, and the activity of the vapors from one cause or another, 

 reach such a height and have so great power that an eruption takes 

 place, — either over the brim of the crater, or through the fractured 

 mountain. The lavas flow off to a distance sometimes of sixty 

 miles or more. Examples are given beyond. 



The outflow of lavas is attended in most volcanoes, as in Vesuvius, 

 with the ejection of cinders, and they continue to be thrown out 

 long after the flow has ceased. They thus build up a cinder-cone 

 immediately around the open vent. 



Most of the small cones about volcanic mountains — called often parasitic 

 cones — are formed in this manner about a point in some opened fissure from 

 which lavas were ejected. Cinder and vapor eruptions are the last effects of the 

 subsiding fires of a volcano. Mt. Kea is an example of a mountain-cone finish- 

 ing its career as an eruptive volcano by the formation of a number of cinder- 

 cones at summit : their height is 300 to 500 feet. In 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 lava has a surface-crust, four to six inches thick, of glassy scoria, which is 

 light and rather fragile. Boiling covers the lavas in the pools with a scum, as 

 it does molasses, and the scoria is the hardened scum or froth. Below this sco- 

 riaceous 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 

 without having undergone any boiling, the stream is often solid lava through- 

 out, 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 clinhers. 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, extensive clinker-fields, 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 evaporated by the heat while the flow was in progress. 



In a submarine eruption, or wherever the lavas enter the sea, an upper por- 

 tion of the outflow, in contact with the water, is shivered to fragments ; if in 

 deep water, the fragments 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, besides spreading far and 

 wide over the adjoining region; or they make a permanent boiling basin, which 



