310 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



" proximately, and gained the result, that according to the known 

 " values of the nutation and precession, the thickness of the solid 

 " crust cannot be less than one fourth or one fifth of the radius 

 " of the earth and must at least amount to 172 to 215 

 "German geographical' miles (=688 to 860 English geogra- 

 f phioal miles.) Such- a thickness of the earth's crust seems indeed 

 " to stand in the necessary relations to the stability of the exterior 

 " surface of the earth, but also almost completely to exclude the 

 " possibility of a communication with the interior of the earth,. 

 " which is really so decidedly shewn to exist by varied volcanic 

 " phenomena. Hopkins also adopts the view that with such a 

 " thick crust a direct communication is impossible between the 

 " interior of the earth and the surface. In order therefore to 

 " explain the phenomena of volcanoes, he supposes the existence 

 " of very large cavities here and there within the solid crust, 

 f which are filled with easily fusible materials,, still in a liquid 

 " state, and which resemble colossal bubbles, enclosing whole seas 

 " of fused substance.* Elie de Beaumont and others, on the 

 other hand,, entertain the view that spaces were formed between 

 the solid crust, and the fluid centre which, at least in earlier geolo- 

 gical periods, caused partial depressions of the earth's crust, and 

 which are still to be considered as the real laboratories of volcanic 

 activity. 



Somewhat allied to Hopkins's supposition is Bunsen's theory, 

 which rests upon certain ascertained facts with regard to the com- 

 position of igneous rocks generally, but more especially to that of 

 lavas. Bunsen supposes the existence in the interior of the earth 

 of two enormous reservoirs of fused matter having each a differ- 

 ent composition, and from the amalgamation of which all the 

 known varieties of trachytic and doleritic rocks result. This theory 

 is based upon two series of analyses of Icelandic lavas, the one 

 comprising, accordingto Bunsen, those richest in silica (trachytes), 

 the other those containing the largest amount of bases, (trap 

 rocks, basalts and basaltic lavas). The first series of analysis com- 

 prised those of the following rocks : — 



1. Trachyte from Baula. 



2. Do from Kalmanstunga. 



. 3.. Do from Langarfja.ll near the Geyser. 



4. Trachyte from Arnar Knipa on the Laxa. 



5. Do from Falklakletturnear Kalmanstunga. 



..^ . — . i 



* NaumauB, Geognoaie, I, T5. 



