OF MONTE SOMMA AND VESUVIUS. 47 
The dykes of Monte Somma that thread through the mass of the 
section of the Atrio in great number, have received special attention 
from Breislak *, Necker, Scropet, Lyell $, and Mallet ||, to whose 
statements little can be added until a complete examination of their 
chemical composition and microscopic structure has been made. 
One peculiarity, however, of important bearing on a point about 
to be discussed is, that nearly every dyke is clothed with a layer or 
saalband of vitreous structure which graduates into the crystalline 
interior, but rarely exceeds one or two centimetres in thickness. 
Lyell 4] remarks that the dykes are not seen on the outer slopes 
of the mountain. This is certainly the case, and for it there are 
many reasons; but space does not permit of discussing them here. 
There are, however, exceptions. For instance, a large one that is 
seen almost perpendicular in the section of the Atrio can be traced 
over the ridge and is exposed for some distance on the outer slope, 
where it traverses obliquely one of the great valleys just east of the 
Punta del Nasone. 
With the exception of the saalband these dykes are composed of 
materials apparently almost identical with those of most lava-streams. 
But they are generally more compact, except at the upper extremity 
near the old surface, where in some cases they are highly vesicular, 
especially in the middle. This condition seems to be explained by 
the small pressure existing allowing the aqueous matter of the un- 
cooled or central part to separate in the form of distinct bubbles, as 
in soda-water on removal of the cork. These bubbles were unable 
to escape entirely from the viscous state of the cooling rock, and 
remain behind so as to give the rock a cellular structure. This 
shows that the water was still in combination with the magma at 
the lower part of the dyke where no bubbles had formed. 
So far we must regard this volcano as having burst through 
various sedimentary strata and volcanic tufas which, except as 
composing the basement or platform for the mountain, form no in- 
tegral portion of it. So far as can be ascertained by the legitimate 
methods of investigation of the geologist, this ancient tufa differs 
entirely from the old leucitic rocks of Monte Somma**, 
The regular superposition of materials, little differing from each 
other, and the regularity with which they were deposited without any 
physical evidence of a great break in time, so far as we can obtain 
* ‘Voyages physiques et lithologiques dans la Campanie,’ 1801. 
+ “Mém. sur le mont Somma.” Mém. de la Soc. de Physique et d’Hist. 
Nat. de Genéye, 1828. 
t ‘ Volcanos,’ 1825 and 1868. 
§ ‘Principles of Geology.’ Different editions. 
|| Quart. Journ. Geol. Soe. vol. xxxiii. 
€ < Principles of Geology,’ 1872, p. 136. 
** T must express great timidity in employing any special name for these 
leucitic rocks. They have been so variously classed and named by different 
authors that although one may have a clear notion of their composition, yet it 
is difficult to decide what term to use for them. As we shall have continually 
to refer to them in our description, Leucilize will be used, simply for the sake 
of brevity, and must not be taken to have any other meaning than either a 
leucitice lava or dyke rock of this voleano. 
