148 
THE CAMBRIAN VOLCANOES 
BOOK III 
vary likewise in texture from somewhat coarse breccias or agglomerates; 
through many gradations, into fine silky schists in which the tulfaceous 
character is almost lost. G-enerally they are distinctly granular, presenting 
to tlie naked eye abundant angular and subangidar lapilli, among which 
broken crystals of a white, somewliat kaoliuized, felspar and fragments of 
fine-grained felsite are often conspicuous. The greater part of the luffs, 
particularly the purple, red and dark-green varieties, which constitute so 
large a proportion of the whole, has been derived from the explosion of 
basic rocks similar in character to the diabases now found associated with 
them. On the other hand, the paler varieties, both in the form of fine 
tuffs and of breccias, have probably resulted mainly from the destruction of 
more siliceous lavas, probably felsites (rliyolites) or other acid rocks. 
That many of the tufts are due to the destruction of diabase -lavas 
may be surmised from their close general external resemblance to these 
rocks, and from the way in which they are associated with the con- 
temporaneous sheets of diabase. Some of the dull dark-purple tuffs 
might almost at first sight be mistaken for truly eruptive rocks. The 
analyses of two typical examples of these basic tuffs (Nos. I. and II.), and 
one (No. III.) of an intermediate variety containing an admixture of acid 
fragments, are given in the subjoined table. 
SiO-2 
AUOa 
Ferns 
FeO 
MuO 
CaO 
MgO 
KoO 
NaoO 
HoO and 
Loss on 
Ignition. 
Total. 
1 
Specific 
Gravity. 
I. 
51-25 
20-41 
3-02 
3-91 
0-21 
4-53 
7-22 
2-93 
1-82 
5-02 
100-32 
2-84 
II. 
48-11 
1.3-30 
3-70 
8-10 
1-43 
8-48 
9-51 
1*57 
1-96 
4-21 
100-37 
2-92 
III. 
61-54 
16-30 
4-40 
3-66 
0-32 
3-08 
2-99 
1-62 
2-81 
2-99 
99-71 
I. Purplisli-red shaly tuff from Ijelow oliviue-diabase, Crag Khosson. Analysis by Mr. J. S. 
Grant Wilson. 
II. Dull purple and green tuff from the lowest group of tuffs between Pen-maen-melyn and 
Pen-y-foel. Analy.sis by Mr. Wilson. 
III. Greenish shaly finely granular tuff, from road-side, north of Board Schools, St. David’s. 
Analysis by Prof. A. Renard of Client. 
Although the majority of the tuffs are more or less basic, they frequently 
contain evidence in the form of small felsitic lapilli that acid lavas were 
present in the eruptive vents, while the pale tuffs show that at the time of 
their discharge it was these acid lavas and not the diabases that were 
blown out by the explosions. Appended are three analyses of the acid 
tuffs (Nos. IV. V. and VI.). 
