336 



PHYSICAL GEOLOGY 



(c) Liquid-thread Theory. 1 — This theory assumes that the earth grew by the slow 

 accession of meteorites (planetesimals), varying greatly in size and composition (p. 

 386), and that the interior, though solid, has become very hot as a result of the com- 

 pression of the interior masses by the accumulation of the outer envelopes. In a globe 



composed of masses varying greatly in 

 composition and fusibility, it is evident 

 that some particles will be molten while 

 others are still solid. As a result of the 

 strains to which the earth is subjected, the 

 liquid portions gradually move toward 

 the surface, uniting in their upward 

 course into larger and larger threads 

 (Fig. 331). Because of their heat these 

 threads finally reach the zone of fracture 

 by fusing and fluxing. When such 

 threads of lava attain the zone of frac- 

 ture, they take advantage of any fissures 

 or Tractures which exist, and are poured 

 out on the surface as fissure or volcanic 

 eruptions. The intermittency of volcanic 

 action is due, according to this theory, 

 to temporary deficiencies in the supply, 

 and the force of expulsion is produced 

 especially by tidal and other stresses and 

 by the slow pressure brought to bear on 

 the threads of liquid rock, until the upper 

 level is reached, where the expansion of 

 the occluded gases begins to operate. 



This hypothesis, as has been pointed 

 out, is based upon the assumption that 

 the earth has never been in a molten con- 

 dition, and that its interior is composed 

 of a heterogeneous mass varying greatly 

 in composition. If the observations upon 



Fig. 331. — Diagram illustrating Cham- 

 berlin's theory of volcanism. S is the 

 surface of the earth ; aa', the zone of frac- 

 ture; af, zone of flow ; jfc, interior portion 

 whose temperature rises from the surface 

 melting point at ff to a maximum at c ; vv, 

 threads or tongues of molten rock rising 

 from the interior to various levels, many 

 of these lodging within the zone of frac- 

 ture as tongues, batholiths, etc. PP are 

 explosive pits formed by volcanic gases 

 derived from tongues of lava below. (After 

 Chamberlin and Salisbury.) 



which the following hypothesis (abyssal 

 injection) is based are well-founded, — namely, that the crust of the earth is com- 

 posed of acid (granitic) rock and that this is underlain by a basic (basaltic) sub- 

 stratum, — the hypothesis cannot stand, since the latter holds that the earth was 

 formerly molten at the surface. 



777. Abyssal Injection Hypothesis 2 



This hypothesis is based both upon laboratory experiments and upon many obser- 

 ve ions of the occurrence and relationships of igneous rocks in various parts of the world. 

 1 1 should be distinctly borne in mind, however, that the hypothesis is as yet unproved. 



l Chamberlin and Salisbury, — Geology, 2d ed., Vol. 1, p. 629. 



2 Daly, R. A., — The Nature of Volcanic Action: Am. Academy of Arts and Sciences, Vol. 

 47, June, uj\ i, pp. 47-122; and Igneous Rocks and their Origin. 



