MECHANISM OF VOLCANIC ACTION—JOHNSTON-LAVIS. 315 
I, Laccolite and sill exposed by erosion at M. 
N Volcano supplied from uncooled part of laccolite, aquiferous rocks, and from 
rift I. 
O Volcano supplied from rift I and aquiferous rocks around top of same. 
P Portion of globe undergoing practically no cooling. 
R Area of ineffective compression or retraction and depression. 
S Area of ineffective contraction and of low pressure. 
Pumices and pumiceous scoria from explosive eruptions of Monte Somma and Vesuvius 
(essential ejecta). 
PrArey 2: 
Fig. 1. Light white pumice, bottom part of Phase III, period 1. The section is 
seen to be mostly composed of a clear glass with only an occasional porphyritic 
erystal or microlith. Magnified 11 diameters. 
Fig. 2. Part of the same section, magnified 50 diameters. Most of the vessels 
are sectionized and open, and support the very rare porphyritic crystal by sec- 
tions of the thin shells of glass. A few vesicles still contain air. 
Fig. 3. Heavy chocolate-brown pumiceous scoria produced later by the same 
eruption, Phase III. Notice how much smaller are the vesicule and how the 
mass of rock material is principally composed of microliths. Magnified 11 
diameters. 
Fig. 4. The same, magnified 50 diameters. Here the numerous large smooth- 
walled vesicles of figure 2 are replaced by few small rough-walled spaces into 
which the microliths project. 
PLATE 3. 
Fig. 5. Light white pumice, bottom part of Plinian pumice that buried Pompeii, 
Phase VII, period 1. The section shows the material to be mostly a clear glass 
crowded by microliths of leucite only with few exceptions. Magnified 11 
diameters. 
Fig. 6. Part of same section, magnified 50 diameters, showing these characters 
more accentuated. 
Fig. 7. Heavy greenish-gray pumice from high up in the pumice stratum. 
Here the rock is composed of a dark, almost opaque network of microlithic 
matter in which magnetite is abundantly distributed. The microliths are 
almost hidden by the opacity of the magnetite and augite grains. Magnified 11 
diameters. 
Fig. 8. The thinnest possible section to make is shown in this figure, magnified 
75 diameters, exhibiting the extensive indivdualization of the glass into opaque 
microliths. The leucite microliths are represented by spots of imperfectly 
transmitted light where the crystal grains reach both sides of the slice, but 
are partially overlapped by augite and magnetite all around.@ 
“Wad this specimen been chosen from the top of the essential ejecta of Phase 
III the thinnest section capable of being cut would have been opaque. It was 
taken, therefore, from a transition stratum (see p. 312). 
