Dr. Johnston-Lavis — Mechanism of Volcanic Action. 439 



of an old volcano, or the initial outburst of a new one, the materials, 

 as they fell from the air, show a definite arrangement, and vary in 

 character in correspondence with different phases of the eruption. 



To illustrate what these characters are I propose to choose a classical 

 example in that of the great outburst of Vesuvius that overwhelmed 

 Pompeii, Herculaneum, Stabia, Oplontis, and other towns around the 

 foot of Somma- Vesuvius. If we examine the deposit of materials 

 that fell during the eruption of a.d. 79 in the streets of Pompeii, or 

 preferably outside the town as the falling houses have disturbed the 

 regularity of the stratification within the walls, we find it made up of 

 several beds. Immediately reposing on the old land surface is a stratum 

 of very white light pumice. I use the word 'white' in comparison 

 with that above it. If we collect a quantity of this we shall see that 

 its bulk is veiy great for its weight. To the naked eye it is composed 

 for the most part of a glassy vesicular base, with here and there 

 scattered crystals of felspars, hornblende, pyroxene, and biotite, 

 besides occasionally extraneous minerals caught up in the magma. 

 Microscopically it is made up of a network of straw-coloured glass, 

 with innumerable minute micro-crystals of leucite, all of a remarkably 

 uniform size, besides which are a very few scattered microliths of 

 hornblende, augite, mica, and felspars, obviously pre-eruptive in birth 

 (PI. XXV, Figs. 5 and 6). At wide intervals, of course, occur the 

 porphyritic crystals above named. The main mass, however, is made 

 up of glass, so that all the vesicles have been able to assume well- 

 rounded outlines. The size of the vesicular spaces is very great in 

 proportion to the amount of solid material enclosing them, making the 

 pumice a very light one in weight. 



Reposing on the bottom stratum, and rather suddenly graduating up 

 from it, is the main bulk of the ejecta. The pumice composing this 

 is much heavier, darker in colour, ranging from a brownish grey to 

 a greenish grey. The porphyritic enclosed ciystals are the same as 

 those in the bottom, white pumice, and practically average the same 

 size. They appear to be more frequent, but this is due to the pumice 

 being denser ; more of them are therefore to be seen in the same area, 

 as it were more crowded together. 



Microscopically this pumice very much differs from the subjacent 

 white variety (PI. XXV, Figs. 7 and 8). Nearly the whole of the glass 

 has been replaced by innumerable microliths. The small leucites have 

 increased but little in size, though they seem more numerous. This 

 I attribute to the less amount of open space left by the vesicles. The 

 augite microlithsconstitutethemain bulk, and minute grainsof magnetite 

 are abundant. Some small microliths are probably distinguishable as 

 felspars. Generally the vesicular cavities are much smaller. The 

 vesicle walls are no longer smooth, but rough from the projecting 

 microliths that in the process of rapid cooling grew and projected in 

 all directions, and are not arranged with such parallelism that is seen 

 in flow-structure with rods already in existence at an earlier date. 



Both these divisions of essential ejecta are more or less mixed with 

 accessory and accidental ejecta torn from the sides of the crater and 

 the sub-volcanic platform. I mention this as it has a bearing on 

 the composition of the third and uppermost part of the materials shot 



