290 DYNAMICAL GEOLOGY. 



conical hill. But volcanic cinders or ashes are often carried by the winds to 

 great distances, and when abundant, make extensive deposits with horizontal 

 bedding ; and such deposits may, in extreme cases, reach a thickness of hun- 

 dreds of feet and bury forests. 



Where small cones have been mostly removed to their base, they may 

 show a central cone of lava — the lava that in its active state was the 

 source of the ashes, and around it more or less of the ejected ashes or lava. 

 Such places have been called '' volcanic necks." 



(d) The earlier lava-streams of a great volcano much thicker than the later. 

 — On going up the valleys of Tahiti (Fig. 161), the thickness of the lava- 

 streams was found by the author to increase from 10, 20, and 30 feet along 

 the coast to 500 and 1000 feet, five or six miles in the interior ; and in Kaxiai, 

 of the Hawaiian group, the same general fact proved to be true. These 

 great volcanoes appear to have poured lavas out copiously at their com- 

 mencement, and to be now in a greatly dwindled condition. In what geolog- 

 ical period the Tahitian and Hawaiian volcanoes began to flow is unknown. 



(e) The interior of the volcanic mountain before and after extinction. — In 

 times of activity, a great volcanic mountain has within it a column of liquid 

 lavas, the lava-conduit, which may be two, three, or more miles in diameter. 

 During the long period of activity the heat of the column spreads far into 

 the adjacent cooled lavas, occasioning in them a more coarsely crystalline 

 condition than that of the modern lava-stream. 



At the extinction of the volcano, if the ascensive force continued to hold 

 the summit of the lava-column to its high position, the enormous liquid mass 

 would have cooled with extreme slowness, and become throughout more or 

 less crystalline. The nearly vertical face of the central peak of Tahiti, 3000 

 feet or more in height, as seen by the author from a summit near by (page 

 180), was found to be without any trace of layers; it was just such a con- 

 tinuous mass from the top down, as the cooling of a lofty, central lava-mass 

 would have made. And rounded stones of a coarsely crystalline granite 

 rock, found along the bed of the stream six to eight miles up one of the 

 valleys, appeared to be evidence as to the crystalline structure of the central 

 peak, sustaining the principle as to the connection of grade of crystallization 

 with rate of cooling. (D., 1839.) 



Extinction is a consequence of a withdrawal of heat, or failure of the ascensive action. 

 But the circumstances attending it may be various. A general collapse or down plunge 

 of the summit at the eruption may leave a crater 2000 feet deep, as in the case of Halea- 

 kala in east Maui, or a collapse may fail to take place at the final eruption, through a 

 gradual decline of heat within, and the mountain hence be left without a visible crater, as 

 is true of Mount Kea. E. D. Preston has proved, by gravity determinations with the 

 pendulum, that Haleakala below its crater is solid, the gravity found being 2-7, and that 

 Kea in its upper part, giving 2-1, is hollow. The same evidence has indicated that the 

 volcanic mountains of Ascension Island, St. Helena, and Fujiyama in Japan, are hollow, — 

 densities of 1-6, 1-9, and 2-1 having been found severally for the masses of these moun- 

 tains ; and by the deviation of the plumb-line of only 7 or 8 seconds by Chimborazo, it is 

 believed to be indicated that this mountain also is hollow. Preston obtained for the lower 



