Lavis — On the Structure of Rocks. 127 



These facts are of extreme interest in relation to volcanic 

 activity. At the enormous pressure and temperature that an 

 igneous water-bearing magma may exist, the dissolved water 

 equals, or exceeds in energy the same weight of steam or gun- 

 powder. We also see that the crater- forming and eruptive power 

 will be in direct proportion to the amount of superheated water 

 existing in the magma ; and crateriform hollows of ten miles in 

 diameter are not difficult of comprehension. In fact, it seems 

 somewhat astonishing that such excavations are not far greater, 

 when we think of the terrific energy that may be stored beneath 

 us in the form of such enormous dykes as those great masses of 

 diabase in the region of the Hudson river. The great difference 

 between the two conditions is, no doubt, that the water in the 

 boiler is perfectly free to evaporate, whereas in an igneous magma 

 it is molecularly scattered through the viscous mass, so that 

 although the energy stored in equal quantities of water in either 

 condition would be the same, the dispersion will be spread over a 

 longer time in the case of the paste, due to retardation of escape 

 in consequence of viscosity. 



In the author's recent researches on the past and present 

 eruptive phenomena of Vesuvius 1 certain important facts were 

 brought out which it has been possible to confirm in a large 

 number of instances in other volcanoes. When this volcano is 

 in a state of chronic activity, with short intervals between one 

 eruption and another, the violence with which the ejections take 

 place is small compared with what occurs after long periods of 

 quiescence. Thus, for instance, during the building up of the 

 old mountain, and again during the last two to three centuries, 

 we find that a very large portion of the products consisted of 

 continuous masses of lava, whereas in the great explosions that 

 excavated the gigantic crater of the Atrio del Cavallo, and which, 

 from the interstratification of vegetable soils, and denudation 

 marks, are proved to have occurred at long intervals apart, are 

 characterized by deposits of spongy pumice, with a total absence 

 of anything but fragmentary products. But in the above case we 

 have not only geological, but even historical, proof ; for we know 

 that at least for many centuries before a.d. 79 this volcano had 



1 Quart. Joum. Gool. Soc, January, 1884. 



