100 IGNEOUS AGENCIES. 



the contact of water with metallic potassium, sodium, calcium, and 

 magnesium, in the interior of the earth. In such definite forms the 

 theory seems far too hypothetical. 



Recent Theories. — 1. Aqueo-igneous Theory. — Accumulation of 

 sediment on sea-bottoms would necessarily produce corresponding rise 

 of isogeotherms, and thus the interior heat of the earth would invade 

 the sediments with their contained waters. The lower portion of sedi- 

 ments 10,000 feet thick would be raised to a temperature of about 260° 

 Fahr., and of 40,000 feet thick (sediments of this thickness and more are 

 known) to that of 860°. This temperature, or even a less temperature 

 if alkali be present, would be sufficient in the presence of the contained 

 water of the sediments to produce complete aqueo-igneous fusion, and 

 probably to develop elastic force sufficient to produce eruption. This 

 view was first brought forward by John Herschel. Observe that this 

 temperature and the corresponding force would be gradually developed 

 as the accumulation progressed, until sufficient to produce these effects. 

 Observe, again, that in this case the water does not seek the heat by 

 descending (the difficulties in the way of this we have already seen), 

 but the heat seeks the already imprisoned water by ascending. 



It seems very probable that cases of eruption of hot mud and of 

 aqueo-igneously fused lavas may be accounted for in this way, but the 

 temperature would not be sufficient to account for true igneous fusion. 



2. Mechanical Theory. — As we shall explain hereafter (p. 263), 

 there is much reason to believe that the interior of the earth is con- 

 tracting more rapidly than the exterior, and that the exterior is thus 

 necessarily thrust upon itself by irresistible horizontal j)ressure. Ac- 

 cording to Mr. Mallet, the crushing of the rocky crust in places under 

 this pressure develops heat sufficient to fuse the rocks, and to produce 

 eruption. But it is at least doubtful whether the heat thus generated 

 would alone be sufficient for this purpose. 



3. Issuing of Superheated Gases. — Rev. O. Fisher has advanced a 

 view which deserves attention. He thinks volcanoes are vents through 

 which issue from the earth's interior superheated steam and gases, 

 melting the rocks in their course and ejecting them by their press- 

 ure. According to this view, the water is not derived from the sur- 

 face, but is original and constituent. This view is independent of 

 the condition of the earth's interior, whether solid or liquid; for a 

 temperature which would permit solidity at great depths would pro- 

 duce fusion under less pressure near the surface.* The sun may be 

 regarded as a globe in an earlier and more active stage of vulcanism. 

 From its interior gases are seen to issue in great quantity, and almost 

 constantly. 



* Cambridge Philosophical Society, 1875. 



