VOLCANOES. 87 



Astronomical Reasons. — There is another and entirely different 

 line of reasoning which has led some of the best mathematicians and 

 physicists to the same result. According to the thin-crust tlieory, the 

 earth is still substantially a liquid globe, and therefore nnder the at- 

 tractive influence of the sim and moon it ought to behave like a yielding 

 liquid. Now, according to Hopkins, Thomson, and others, the earth in 

 all its astronomical relations behaves like a rigid solid — a solid more 

 rigid than a solid globe of glass — and the difference between the be- 

 havior of a liquid globe and a solid globe could easily be detected by 

 astronomical phenomena.* A complete exposition of the proof w^ould 

 be unsuitable to an elementary work. Suffice it here to say that the 

 force of these arguments has led some geologists to the conclusion that 

 the earth must be regarded as a substantially solid and very rigid globe ; 

 that volcanoes are openings into local reservoirs of liquid, not into a 

 general liquid interior — into subterranean fire-lakes, not into an interior 

 fire-sea ; and that, therefore, the theories of igneous phenomena must 

 be constructed on the basis of a substantially solid, not a substantially 

 liquid, earth. 



There are many phenomena, however — esjoecially the great lava- 

 floods — to be described hereafter and the instability of the crust-level 

 under increase and decrease of weight by sedimentation and erosion, 

 which seem to require an unlimited supply of liquid matter at no great 

 distance beneath the surface. Many geologists, therefore, find a com- 

 promise in the view that there exists a liquid or semi-liquid lager, 

 either universal or over large areas, between the solid crust and a solid 

 nucleus. This is called the sub-crust layer. This seems, on the whole, 

 the most probable view.f 



The interior heat of the earth manifests itself at the surface in three 

 principal forms, viz., volcanoes, earthquakes, and gradual oscillations of 

 the eartWs crust. 



Section 2. — Volcanoes. 



Definition. — A volcano is usually a conical mountain, with a funnel- 

 shaped, or pit-shaped, or cup-shaped opening at the top, through which 

 are ejected materials of various kinds, always hot, and often in a fused 

 condition. The activity of volcanoes is sometimes constant, as in the 

 case of Stromboli, in Italy, and Kilauea, in Hawaii, but more commonly 

 intermittent, i. e., having periods of eruption alternating with periods 

 of more or less complete repose. Volcanoes which have not been 

 known to erupt during historic times are said to be extinct. It is im- 

 possible, however, to draw the line of distinction between active and 



* Thomson has recently reaffirmed these conclusions with still greater positiveness. — 

 Nature, vol. xiv, p. 426 ; American Journal of Science and Art, vol. xii, p. 336, 1876. 

 f American Geologist, vol. i, p. 382, vol. ii, p. 28, and vol. iv, p. 38. 



