On the Agency of Water in Volcanic Eruptions, 133 



movements, and no persistent issue of steam gives countenance to 

 the supposition that the water permeates the rocks to great depths or 

 exists there in natural cavities. 



Natural cavities at depths in the earth's crust I hold to be impos- 

 sible. There may be cavities in the Igneous rocks near the surface, 

 due either to contraction, to rapid cooling without pressure, or to the 

 shell left by the escaping lava streams. But these cannot take place 

 at great depths. They are connected with subaerial action. 



With regard to such cavities as those so common and of such 

 extent in limestone rocks, it must be remembered that these cavities are 

 entirely due to the descent of the surface waters to a definite level, 

 and to their escape by the most readily available outlet, either in adja- 

 cent valleys, or at or near the tide line on the adjacent coast. Below 

 that level there can be no active circulation of water, and no possi- 

 bility, therefore, of great cavities, due to the passage of water 

 through underground channels, being formed. Changes of level 

 may have carried some of these superficial cavities to certain depths 

 beneath the surface, but that they should have been carried to the 

 great depths we are referring to, or be of any sufficient size, is more 

 than problematical. In limestone strata they occur near the surface, 

 or at a short distance beneath the surface ; wherever these rocks have 

 been worked at a depth beneath the line of water saturation, such 

 cavities are of very rare occurrence. Deep mines reveal occasionally 

 a few fissures, and some comparatively small cavities, but these 

 are in mineral veins, which show no relation with active volcanic 

 phenomena. 



§ 3. Influence of Volcanic Eruptions on Spring and Well Waters. 



It is a singular circumstance that although the presence of water in 

 volcanic eruptions has been so long recognised, and the disturbances 

 caused to wells and springs have been so often noticed, no systematic 

 series of observations has been made either on the surface or on the 

 underground waters in connexion therewith. There are many allu- 

 sions and incidental notices, but nothing in the form of special and 

 exact details. Most writers on the subject speak of the disturbances 

 to wells and springs as a common or obvious fact; but a series of 

 extended and accurate observations is much needed.* In the absence 

 of more exact data, we have to avail ourselves of general observations 

 made by witnesses on the spot, amongst whom are many competent 

 authorities. 



The great eruption of Vesuvius of 1813-14, which commenced with 

 a few trifling explosions and shocks in September, and by a small 

 eruption of lava in October and November, followed by the great 



* The observations should not be limited to the volcanic area, but should extend 

 to the sedimentary strata around, and to some distance from the centre of eruption. 



