34 
VOLCANIC PRODUCTS 
liOOK I 
from which they were derived. But their frequently decayed condition 
makes it less easy, in their case, to draw definite boundary-lines between 
varieties. In a group of acid lavas, the tuff's may be expected to he also 
acid, while among intermediate or basic lavas, tlie tuffs will generally be 
found to correspond. There are, however, exceptions to this general rule. 
As will he afterwaixls described in detail, abundant felsitic tuffs may be 
seen among the andesitic lavas of Lower Old Bed Sandstone age in Scotland, 
and rhyolitic tuffs occur also among the Tertiary basalts of Antrim. 
As a rule, basic and intermediate tuffs, like the lavas from which they 
have been derived, are rather more prone to decomposition than the acid 
varieties. One of their most characteristic features is the presence in them 
of lapilli of a minutely vesicular pumice, which will be more particularly 
described in connection with volcanic necks, of which it is a characteristic 
constituent. Occasional detached crystals of volcanic minerals, either entire 
or broken, may be detected in them, though perhaps less frequently than 
in the agglomerates. The earthy matrix is generally greenish in colour, 
varying into shades of lirick-red, purple and brown. 
The acid tuffs are, on tlie whole, paler in colour than the others, some- 
times indeed they are white or pale grey, but graduate into tones of 
hicmatitic red or brown, the varying ferruginous tints being indicative of 
stages in the oxidation of the iron-bearing constituent minerals. Small 
rounded lapilli or angular fragments of felsite or rhyolite may be noticed 
among them, sometimes exhibiting the most perfect how-structure. As typical 
examples of such tuffs, I may refer to those of the Pentland Hills, near 
Edinburgh, and those that lie between the two groups of basalt in Antrim. 
Thrown out promiscuously from active vents, the materials that form 
tub's arrange themselves, on the whole, 
according to relative size over the surface 
on which they come to rest, the largest 
being generally grouped nearest to the 
focus of discharge, and the hnest extending 
farthest from it. As the volcanoes of which 
records have been preserved among the geo- 
logical formations were chiehy subaqueous, 
the fragmentary substances, as they fell into 
water, w'ould naturally be to some extent 
spread out by the action of currents or 
waves. They would thus tend to take a 
more or less distinctly stratihed arrangement. 
Fkj 13. — Alternations of coarser ami liner waVCS 
Tuff. 
Moreover, as during an eruption there might be successii'e paroxysms of 
violence in the discharges, coarser and finer detritus would successively 
fall over the same spot. In this way, rapid iilternatious of texture 
would arise (Fig. 13). A little experience will enable the observer to 
distinguish between such truly volcanic variations and those of ordinary 
sedimentation, where, for inst;ince, layers of gravel and sand repeatedly 
alternate. Besides the volcanic nature of tlie fragments and their non- 
