ORIGIN OP ERUPTIVE AND PRIMARY ROCKS. 333 



considerations. The difference in the compressibility of fluid and 

 solid bodies does not seem to be very considerable. Water is 

 but slightly compressible. According to Oerstedt, the compres- 

 sion produced by a pressure of 2000 atmospheres amounts ouly to 

 1-1 2th.,* and one would suppose that fluid lava would be even less 

 compressible than water. The decrease of compressibility which 

 may accompany solidification would therefore seem inadequate to 

 the production of such stupenduous effects as are observable dur- 

 ing volcanic eruptions. Further, if this were the cause of the 

 ejection of the lava, the latter would be poured forth only by vol- 

 canoes of inconsiderable height, but by these simultaneously. It s 

 ejection would also keep pace with the very slow and gradual 

 solidification in the interior, and violent volcanic paroxysms would 

 not occur. 



Sartorius Von Waltershausen likewise assumes, that expansion 

 takes place, but he does not attribute it to the mere difference in 

 the compressibility of the igneous material before and after soli- 

 dification. He supposes that the expansion takes place in the act 

 of crystallization i, e. while the various minerals form and separate 

 themselves from the fluid magma.f He fails however to adduce 

 any conclusive evidence in support of this supposition, which it 

 might be possible to sustain, in the event of its being possible to 

 show that melted rock rapidly cooled to a fine grained crystalline 

 mass, had a lesser specific gravity than die same slowly cooled 

 and distinctly crystallized. He indeed shows that the specific 

 gravities of the minerals which result in the cooling of igneous 

 rocks, are invariably less than those which, result in calculating 

 &eir specific gravities from the quantities and densities of their 

 constituents ; as the following instances show : — 

 Substance* Density. 



by experiment, from calculation. 



1. Anorthite from Selsfjall 



2. Labradorite from Egersund 



3. Orthoelase from Baveno 



4. Augite from Monte Rosso 



5. Hornblende from Mtua. 



6. English crownglass 



7. Guinands Flintglass 



8. Bohemian glass 



* Gmeliu, Hand-book of Chemistry, II. 62. 

 f Uber die vulcanishen Gesteine. etc., p. 333. 



2. 7 



3.225 



2.705 



3.212 



2.555 



2.935 



2.886 



3.208 



2.893 



3.447 



2.487 



2.721 



3.77 



5.64 



2.396 



2.735 



