302 Dr. Johnston-Lavis — Vesuvius and Monte Somma. 



work, these too if they were susceptible of conversion into serpen- 

 tine might be so converted, and we should then have intrusive 

 serpentine as well as interbedded serpentine, not merely in neigh- 

 bouring localities, but actually in the same locality. 



Assuming the intrusive nature of the Coverack rock, it may be that 

 such has been its origin, and such has been certainly the origin of 

 the serpentinous rocks of Clicker Tor, and other localities, to be 

 hereafter referred to. I must here repeat, however, that I have been 

 unable to find any evidence of the existence of intrusive serpentine 

 rock at Porthalla, Mullion, or Kynance. 



IV. — Some Speculations on the Phenomena suggested by a 

 Geological Study of Vesuvius and Monte Somma. 



By H. J. Johnston-Lavis, M.D., F.G.S., etc. 



DISEEGAEDING for convenience the internal volcanic forces of 

 the earth, let us take for granted that we have an unlimited 

 supply of fused silicates at a very high temperature ; what will be 

 the changes and effects produced in the escape of such matters upon 

 the earth's surface ? 



That, at the great sources of volcanic material the igneous mass is 

 in a completely fused state there can hardly exist any doubt. By this 

 is meant that the magma is an intimate mixture of various chemical 

 elements and compounds that have not yet assumed any other than 

 the fluid state, or, in other words, that the molecules move freely 

 amongst each other and have not arranged themselves in any definite 

 crystalline form. 



One of the most important questions that first arises, and is not 

 susceptible of complete response, is whether this fused magma con- 

 tains aqueous matter in " solution " ? ^ 



If we must really regard granite as a primary igneous rock, the 

 presence of water cavities in the quartz might certainly seem to 

 prove the existence of that compound in the original fused magma. 

 We must nevertheless remember that such may have been absorbed 

 from the aquiferous strata in the neighbourhood of the granite if in- 

 trusive ; or supposing such a rock to be the original earth's crust, the 

 question carries us beyond safe speculation. Even admitting that a 

 small portion of aqueous matter is mixed in the fused magma, it is 

 hardly probable that it is sufficient to account for the large amount 

 of vapour escaping during explosive and even tranquil volcanic 

 activity. 



If we take 25 kilometers as the thickness of the earth's solid 

 crust traversed by the igneous matter before its exit, we shall have 

 made but a very moderate estimate. If we make a rough calculation 

 of what amount of material can be contained within a volcanic 

 chimney 25 kilometers long together with its ramifications, with 

 a diameter equal to any of the cores seen in denuded examples of 

 old volcanoes, we shall see that the quantity is very great. 



Let us suppose the area of the section of the chimney equal the 

 ^ By " solutiou " is meant a condition similar to the solution of CO'^ in water. 



