Notices of Mejnoirs — Causes of Volcanic Action. 31 



by radiation into space. Besides, it is evident that even tlie escape 

 of liquid lava and steam from volcanos, and of hot springs from 

 these and other sources, must bring, in however small a quantity, a 

 certain increment of heat from the interior to the surface, where it 

 is lost. This should lead to a certain contraction at depths, and of 

 re-adjustment of the external crust, in consequence of which the 

 fused masses of the interior will from time to time tend to be forced 

 outwards, whenever tension became sufficient to overcome resistance. 

 In this the author agrees with many other geologists. The further 

 hypothesis respecting volcanic action which he now suggests, he 

 has, however, been mainly led to form by his researches on under- 

 ground waters, and may be stated in a few words as follows : — 



A portion of the rain falling on the surface not only of permeable 

 and fissured sedimentary strata, but also of fissured and creviced 

 crystalline and other rocks, passes below ground, and is there 

 transmitted as far down as the permeable rocks range, or as the 

 fissures in the rocks extend, unless some counteracting causes inter- 

 vene. Those causes are the occurrence of impermeable rocks, faults, 

 and heat. The former two are exceptional, the latter constant. The 

 increase of temperature with depth being 1° Fahr. for every 50 to 

 60 ft., the boiling-point of water would be reached at a depth of 

 about 10,000 ft., but owing to the pressure of the superincumbent 

 rocks, it has been estimated that wateT will retain its liquidity and 

 continue to circulate freely to far greater depths. Unfortunately, 

 very little is known of the substrata of volcanos. Etna and Hecla 

 apparently stand on permeable Tertiary strata, Vesuvius on Tertiary 

 and Cretaceous strata, while in South America some of the volcanos 

 are seemingly situated amongst PalEeozoic and crystalline rocks. 

 Under ordinary circumstances, all the permeable strata and all 

 fissured rocks become charged with water up to the level of the 

 lowest point of escape on the surface, or if there should be an 

 escape in the sea-bed, then to that level, plus a difference caused 

 by friction. 



The extreme porosity of lavas is well known. All the water 

 falling on the surface of Etna and Vesuvius (except where the 

 rocks are decomposed and a surface soil formed) disappears at once, 

 passing into the fissures and cavities formed by the contraction of the 

 lava in cooling. Not only are these fissures filled, but the water 

 lodges in the main duct itself, and occasionally rises to a height to 

 fill the crater. Beneath the mass of fragmentary and cavernous 

 volcanic materials forming the volcano, lies the original compact 

 mass of sedimentary strata, etc. Owing to the fortunate circum- 

 stance of an artesian well having been sunk at Naples, we know 

 the underlying sedimentary strata there to consist of alternating 

 strata of marl, sands, and sandstones, some water-bearing, others 

 impermeable. The water from the lowest spring reached in this 

 boring, rose at first 8 feet above the surface, and 81 feet above the 

 sea-level. Where the strata crop out in the sea-bed, the same 

 pressure of the column of inland water forces the fresh water 

 outwards, so as to form a fresh water spring in the sea, as at 



