Jounston-Lavis—The Eruption of Vesuvius in April, 1906. 165 
the fluid magma would slowly ooze along a narrow rift, exposed to hardly any 
mechanical disturbance, partially but slowly cooling under some pressure. The 
conditions were such as to favour, not the sudden bringing into existence of a 
great number of leucites, felspar, magnetite, or augite microliths, but the gradual 
increment of those already in existence. I demonstrated years ago, that the rival 
minerals leucite and some of the felspars depended for their production on 
whether there was great or little pressure, and the rapidity of cooling.* Felspars 
are the slow, more stable, or if I may say so, dignified results of cooling of 
an igneous rock, whilst the felspathoids represent the more hasty, hurried 
products. The latter condition was markedly the case with the rapid outpours 
of lava during the April eruption, although the weeks of cooling of the more 
homogeneous and bulky lava-streams would partly repair the agitation in the 
products of the parturient mountain; the vast number of leucite microliths had 
already been called into existence, each claiming only its limited share of suitable 
substances for its growth. 
We have a striking proof of the large percentage of glass that is differentiated 
or microlithized in the cooling of these lava-streams at the time of their emission 
in the following fact: Some of the rapidly flowing lava ran into a deep cistern 
of water at a villa at Boscotrecase, by which a highly glassy, fragile mass was 
produced, of a deep-black, vitreous appearance. Under the microscope, the 
average size of the leucite microliths is smaller (figs. 22 and 23, Pl. XVII.) than 
in the interior of the lava-stream close at hand, which cooled less rapidly. In 
the rapidly cooled rock the ground-mass is far more transparent, is largely made 
up of greenish-buff glass, with the smaller leucite, felspar, and augite microliths 
much smaller and more sparsely distributed (compare figs. 23 and 25). The iron 
ore grains are fairly well shaped and, no doubt, represent those already formed 
before the sudden cooling by the water. This cooling was so rapid that it 
prevented the ground-mass from becoming dirty and opaque by the liberation of 
fine magnetite dust, as can be found in the surface of the scoriaceous crust of 
these same lavas. ‘The microliths are, though very small, very well formed in 
this glassy ground-mass. Many of the augites show their prismatic, pinacoidal, 
and domic faces to great perfection, if the focussing screw is kept moving. The 
lozenge-shaped felspar crystals are equally neat, and quite similar to those 
described and figured in my account of the 1885 eruption.t 
In the following table, we have two analyses of the lava. which are sensibly 
the same, and quite within the variations in any lava-flow. For the sake of 
* “The Relationship of the Structure of Igneous Rocks to the Conditions of their Formation.”— 
Scient. Proc. Roy. Dublin Soc., vol. v. (N.S.), 1886, pp. 112-156. 
+ ‘‘L’ Eruzione del Vesuvio nel 2 Maggio, 1885.—Ann. d. Accad. O. Costa d. Aspiranti Naturalisti 
Era 3, vol. i., Napoli, 1886, p. 8, with chromolithog. 
2D 2 
