CHAP. Ill 
NATURE OF TUFFS 
35 
water-worn forms, he will notice that here and there the larger blocks may 
he placed on end — a position the reverse of that usual in the disposal of 
aqueous sediments, but one which is not infrequently assumed by ejected 
stones, even when they fall through some little depth of water. Further, 
the occurrence of large pieces of lava, scattered at random through deposits 
of fine tuff, would lead him to recognize the tumultuous discharges of a 
volcanic focus, rather than the sorting and sifting action of moving water. 
Admirable illustrations of these various characteristics may be gathered 
in endless number from the I’aheozoic volcanic chronicles of Britain. 1 may 
especially cite the liasin of the Firth of Forth as a classical region for the 
study of Carboniferous examples. 
When the conditions of modern volcanic eruptions are considered, it will 
be seen that where ejected ashes and stones fall into water, they will there 
mingle witli any ordinary sediment that may be in course of deposition at 
the time. There will thus l)e a blending of volcanic and non - volcanic 
detritus, and the transition between the two may l)e so insensible that no 
hard hire of demarcation can lie drawn. 
Such intermingling has continually taken 
place during past ages. One of the first 
lessons learnt by the geologist, who begins 
the study of ancient volcanic records, is 
the necessity of recognizing tliis gradation 
of material, and likewise the frequently 
recurring alternations of true tuff with 
shale, sandstone, limestone or other entirely 
lion-volcanic detritus (Fig. 14). He soon 
perceives that such facts as these furnish 
him with some of the most striking proofs 
of the reality and progress of former 
eruptions. The intermingling of much ordinary detritus with the volcanic 
material may be regarded as indicative either of comparatively feeble ac- 
tivity, or at least of considerable distance from the focus of discharge. It 
IS sometimes possible to trace such intermixtures through gradually augment- 
ing proportions of volcanic dust and stones, until the deposit becomes wholly 
volcanic in composition, and so coarse in texture as to indicate the prox- 
imity of the eruptive vent. On the other hand, the gradual decrease of the 
volcanic ejections can be followed in the upward sequence of a series of 
stratified deposits, until the whole material becomes entii'ely non-volcanic. 
The occurrence of thin partings of tulf between ordinary sedimentary 
strata points to occasional intermittent eruptions of ashes or stones, the 
Vigour and duration of each eruptive interval being roughly indicated by the 
thickness and coarseness of the volcanic detritus. The pauses in the volcanic 
activity allowed the deposit of ordinary sediment to proceed unchecked. 
-The nature of such non-volcanic intercalations gives a clue to the physical 
conditions of sedimentation at the time, while their thickness affords 
some indication of tlie relative duration of the periods of volcanic repose. 
Tn^rri iTOiniTn ^ 
a Q ^ ® “■»» OT O • „ I" • . ^ 
v>. O O*^- •* -O 
Fig. 14. — Alternations of Tuff (/, ^,) with 
nou-volcanic sediment (/, V). 
