36 
VOLCANIC PRODUCTS 
BOOK 1 
A little reflection will convince the observer that in such a section as 
that represented in Fig. 14 the volcanic intercalations must he regarded as 
a mere local accident. Evidently the normal conditions of sedimentation at 
the time these strata were accumulated are indicated by the limestone bands 
(/, 1). Had there been no volcanic eruptions, a continuous mass of limestone 
would have been deposited, but this continuity was from time to time 
interrupted by the explosions that gave rise to tlie intercalated bands of 
tuff ($, t). 
The application of these rules of geological evidence will be best under- 
stood from actual examples of their use. Many illustrations of tliem^ will 
be subsequently given, more especially from the volcanic records of the 
Carboniferous peifod. 
One of the most interesting peculiarities of interstratified tuffs is the 
not infrequent occurrence of the remains of plants and animals imbedded 
in them. Such remains would have been entombed in the ordinary sedi- 
ment had there been no volcanic eruptions, and their presence in the tuffs 
is another convincing proof of contemporaneous volcanic action during the 
deposition of a sedimentary series. But they may be made to furnish much 
more information as to the chronology of the eruptions and the physical 
geography of the localities where the volcanoes were active, as will be set 
fortli farther on. 
Tuffs, as already remarked, frecpiently occur without any accompani- 
ment of ’lava, although lavas seldom appear without some tuff. We thus 
learn that in the past, as at present, discharges of fragmentary materials 
alone were more common than the outflow of lava by itself. The relative 
proportions of the lavas and tuffs in a volcanic series vary indefinitely. In 
the Tertiary basalt-plateaux of Britain, the lavas succeed each other, sheet 
above sheet, for hundreds of feet, with few and trifling fragmental intercala- 
tions. Among the Carboniferous volcanic ejections, on the other hand, many 
solitary or successive bands of tuff may be observed without any visible 
sheets of lava. Viewed broadly, however, in their general distribution 
during geological time, the two great groups of volcanic material may be 
regarded as having generally appeared together. In all the great volcanic 
series, from the base of the Baheozoic systems up to the Tertiary plateaux, 
lavas and tuffs are found associated, much as they are among the ejections 
of modern volcanoes. They often alternate, and thus furnish evidence as to 
oscillations of energy at the eruptive vents. 
Now and then, by the explosions from a volcano at the present day, a single 
stone may be ejected at such an angle and with such force as to fall to the 
ground at a long distance from the vent. In like manner, among the volcanic 
records of former periods, we may occasionally come upon a single block of 
lava imbedded among tuffs or even in non-volcanic strata. Where such a 
stone has fallen upon soft sediment, it can be seen to have sunk into it, 
pressing down the layers beneath it, and having the subsequently deposited 
layers heaped over it. An ejected block of this nature is represented among 
the tuffs shown in Fig. 13. Another instance from a group of non-volcanic 
