282 DYNAMICAL GEOLOGY. 



crust 1 or 2 inches thick. Beneath it, the most recent lava is comparatively solid 

 and often columnar in structure. The outside lavas of the mountain which have been 

 ejected through fissures have no such crust, but sometimes a solid glassy exterior of a 

 fourth to half of an incli. 



Volcanic glass usually contains moisture enough for making it a light scoria when 

 it is heated to fusion before the blowpipe, as shown by J. W. Judd (1886), and also by 

 Iddings, in the case of the Yellowstone Park obsidian. 



(c) Rupturing and other effects from expansive action. — Vapors may also 

 produce fractures in the walls of craters or in the sides of the volcanic 

 mountain by their sudden generation within regions about or beneath the 

 crater, and also by their slow accumulation within confined spaces, and thus 

 may occasion volcanic eruptions. They produce the most violent projectile 

 effects when water in large quantity gains direct access to the lava-conduit ; 

 for the conditions are then those that cause the most violent of boiler-explo- 

 sions, except that they are on a scale as much greater as the volcano is 

 larger than a boiler. 



Vapors also bring pressure to bear on surfaces of liquid lava beneath 

 them, and force the lava up fissures to levels hundreds of feet above the 

 bottom of a crater. 



The vapors are thus the chief source of power in the volcano. They may 

 work quietly, but they are at the bottom of all violent work. 



(d) Vapors of deep-seated origin. — While all the ordinary projectile 

 work of volcanoes may be carried forward by vapors from waters that gain 

 access from the sea, or the fresh waters of the land, it is a question whether 

 vapors from the deep-seated source of volcanic action may not have aided 

 explosively the first opening of the volcano. The lifting action of the 

 ascensive force in Kilauea is so quiet, and its progress so slow, — 400 feet 

 at the most in six years, — that we have no favorable answer from this 

 source. Daubree has experimented on the effects of steam, driven under 

 high pressure along a fracture in blocks of granite, and proved the efficiency 

 of such a course in making a tubular passage through it. The results are 

 published in a volume entitled, Les Regions invisibles du Globe et des 

 JEspaces celestes, 1892, and in earlier papers read before the French Academy. 



The occurrence of volcanoes in long lines implies dependence as to origin on great 

 fractures, and mutual dependence of the volcanoes along any such line. The lines are 

 often in' parallel ranges or series of fissures, and must have opened through the earth's crust 

 to the depths that supplied the melted rock. In some cases the volcanic action along such 

 lines has continued longer at one end of the line, or of the several lines in a series, than 

 at the opposite end, and extinction has been in like manner serial. An example is afforded 

 by the Hawaiian group. The group, now so called, is about 400 miles long and west- 

 northwest in trend. The islands consist either of a single volcano, or of two or more 

 united. The prominent doublets are Oahu and Maui ; and Hawaii is a quintuplet, in two 

 lines. The map of Maui, on page 179, shows plainly by the aged appearance of its erosion 

 over west Maui, that this western of the two volcanoes long since became extinct, while 

 east Maui has the smooth face of youth and may have been active within two or three 

 centuries. There is the same evidence that west Oahu was extinct long before east Oahu, 



