152 Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 
of lava. Vesuvius, in the whole of its history, either recorded by man or 
decipherable from its rocks, gives no evidence of any other grades of activity than 
those which I have pointed out during the last quarter of a century, and which 
are marked* on the following scale :— 
ScHEME oF GRADES AND VARIETIES or AcTIVITY IN VOLCANOES. 
* 1,—WMephitic (acid and basic). 
* 2,—Fumarolic (acid and basic). 
* 
3.—Solfaratic (acid ; rarely basic). 
| 4a.—Ordinary chronic activity ; lavas rare. 
Ci Staal (neato) 4b.—Paroxysmal outbursts very rare. 
5a.—Ordinary ; layas abundant. 
Go sVORIIED (WERE) oo os | oe eas frequent. 
6a.—Quiescent. 
6.—Paroxysmal, with great outflows of lava usually. 
* 
6.—Icelandic (basic; rarely acid) 
| 7a.—Solfaratic or fumarolic. 
7.—Vuleanian or Pelean (acid) 1B. easel 
8.—Plinian or Explosive (acid or basic). (Pumice.) 
9.—Phlegren (acid; more rarely basic). Production of a volcano, usually explosively by a 
single eruption, or rarely more than one at a long interval. 
The present eruption, therefore, cannot be better described than of the 
paroxysmal Vesuvian degree following upon an ordinary Vesuvian degree. 
The more I study active volcanoes the more I become convinced that the 
actual output of material for any given one is sensibly uniform and continuous 
over long periods, but of course most variable for short ones. That uniformity is 
not so marked if our observations are limited to a short period. Years ago 
I demonstrated that by the study of Vesuvius and its products we there had 
admirably illustrated the fundamental principles of volcanie action. I have demon- 
strated that in what we include under that term there are two groups of 
phenomena quite different in themselves, but so linked together that vulcanologists, 
with few exceptions, have failed to unravel them. 
It is by the incontrovertible records left in the structure and composition of 
the volcanic products, that it is relatively easy to interpret these various phenomena. 
Although it is a quarter of a century ago since those principles were laid down, 
volumes of unsupported hypotheses have been launched on the public.* This 
great misfortune is doubtless attributable to the persistence of petrographers 
* “The Geology of Monte Somma and Vesuvius,’’ Quart. Journ. Geol. Soc., vol. xl., 1884; and 
‘“‘The Relationship of the Structure of Rocks to the Conditions of their Formation.’”’—Sci. Proc. Royal 
Dublin Society, vol. y.(N.8.), 1886. ‘‘The Causes of Variation in the Composition of Igneous Rocks.”— 
Natural Science, vol. iv., 1894, p.134. ‘Extension of the Mellard Reade and C. Davison Theory of 
Secular Straining of the Earth to the Explanation of Deep Phenomena of Volcanic Action.”—Geol. Mag., 
Dec. u1., vol. vii., 1890. 
