624 GEOLOGY. 



this view, (1) the one postulating a general molten interior, (2) the 

 other limiting the molten matter to local reservoirs. 



Hypothesis i. Lava outflows from a molten interior. — In the early 

 days of geology, when the earth was supposed to have a thin crust and 

 a molten interior, it was very naturally assumed that volcanoes were 

 but pipes leading down to the molten mass within. This view has been 

 essentially abandoned. The independence of adjacent vents is in itself 

 almost a fatal objection, when it is recalled that the height of recent 

 volcanic craters ranges from nearly 20,000 feet above the sea, to 10,000 

 to 20,000 feet below. The view would involve the conception of lava- 

 columns connected with a common reservoir varying possibly 30,000 to 

 40,000 feet in altitude, and certainly more than half that much, simul- 

 taneously. The lower outlets should as certainly be selected for the 

 outflow of the great interior sea of fluid rock, as the lowest sag in 

 the rim of a lake for its outflow, for no great differences in specific 

 gravity are presumable under this hypothesis. An equally grave objec- 

 tion arises from tidal strain. If the earth were liquid within and merely 

 crusted over by a shell of rock of moderate thickness, it would yield 

 appreciably to tidal stresses, and this yielding would change the capacity 

 of the interior so that with every distortion of the spheroid a portion of 

 its fluid interior would be forced to the outside, and with every return 

 to the more spheroidal form there would either be a re-flow to the interior 

 or a shrinking of the crust. In any case a very demonstrative response 

 to tidal influence would tell the story of interior fluidity. No such 

 effects are observed. The tidal strains may perhaps have a slight effect 

 in hastening a given eruption when the forces are approaching a deli- 

 cate balance and an eruption is imminent, but the very triviality of this 

 influence implies not only the absence of a general liquid interior, but 

 also of extensive reservoirs. 



Hypothesis 2. Lavas assigned to molten reservoirs. — A modifica- 

 tion of the preceding view has been made to escape the difficulties 

 involved in the hypothesis as stated above. It is supposed that while 

 nearly all the subcrust solidified, numerous liquid spots were left 

 scattered through it. This honeycombed substratum is supposed to 

 connect the continuous outer crust with a central solid body, solidified 

 because of pressure in spite of its high temperature. This hypoth- 

 esis escapes only a portion of the objections. For instance, the lavas 

 in Mauna Loa and Kilauea in Hawaii differ nearly 10,000 feet in height^ 



