46 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
One of the most beautiful examples of the parallelism between past 
and present is to be seen in the Vallone Genzano, and its next fellow 
to the west. A lava stream has been quarried into, showing a section 
of the flow, which is covered by a rolled and corded surface exactly 
like what we see on that of 1858. The old one looks quite as 
fresh as the modern, although the former is buried by thick beds of 
pumice and tufa of great antiquity. Such perfect preservation of 
flow-surfaces of prehistoric lavas of Somma is by no means un- 
common, and there is hardly a valley without at least one or two 
examples. 
As in the escarpment of the Atrio, so in the valleys, especially the 
deeper ones, on the slopes, we have the same superposition of lava 
streams and their fragmentary equivalents, sometimes in such 
abundance as to be uncountable. Lyell* remarked the fact that 
especially in these valleys the sections showed one set of deposits 
resting unconformably upon the denuded surfaces of those of 
another. It must be remembered, however, that this is much more 
common near the base or toe of the mountain than higher up, as 
would naturally be expected. This erosion seems never to have 
been very extensive, and is no proof of intermittence of eruptive 
activity ; since often long periods intervene between one violent | 
eruption and another, giving time for considerable erosive action on 
such incoherent materials with a highly inclined surface. 
Of the great lava-fHows that have reached lower levels or the 
plains, most are more or less decomposed at their surface and for 
some depth beneath it, so that the leucite is kaolinized and the rock 
reduced to a crumbly mass, which if the stream be thin will involve 
the whole of its thickness. Good examples may be seen at Cisterna 
and near the Pension Soleil beyond Pompeii. 
The section of the Atrio del Cavallo is described by Scacchi + 
thus :—‘* The aggregate is composed of volcanic sand, fragments of 
the same kind of basalt that forms continuous masses, and often of 
loose crystals of augite. At some places it appears to have been 
exposed to high temperature at a later period, so that its elementary 
parts are fused together, and thus it is difficult to distinguish from 
the rock intact.” We have here an accurate description com- 
prised within a few words. At the lower and central portion 
of the section the fragmentary materials are so completely and 
firmly packed together by heat as above described, and also no 
doubt by pressure, that the rock is quite continuous in structure, so 
much so in fact, that in breaking out a specimen the fracture passes 
regularly through the mass. The origin of the materials, however, 
is exhibited by the difference in colour, vesicularity, and crystalliza- 
tion of the components. 
The difficulties that would arise in determining the origin of such 
a rock, if exposed to altering and metamorphosing influences during 
long periods of time, may well be imagined. Many of the ancient 
English volcanic rocks present such enigmatical examples. 
* Principles of Geology,’ 1872, p. 636. 
+ *Qezioni di Geologia, Napoli, 1843, pp. 168 and 169. 
