8o 
THE PERMIAN VOLCANOES 
BOOK VII 
Andrews. He observed the same substance at the Giant’s Causeway, both 
in the basalt and scattered through one of the interstratified beds of red 
bole. Much larger rounded masses of a similar augitic glass, but with a 
distinct trace of cleavage, have already been referred to as occurring in a 
volcanic vent of Upper Old Red Sandstone age at John q Groat s House. 
Biotite is not a rare mineral in some tuffs. It may be obtained in 
Lower Carboniferous tuffs of Dunbar, in plates nearly an inch broad , but 
the largest specimen I have obtained is one from the same Llie vent which 
yielded the large felspar fragment. It measures 2jr X - X ^ inches, lliese 
mica tables, like the other minerals, are abraded specimens. 
That these various minerals were ejected as fragments, and have not 
been formed in situ, is the conclusion forced upon the observer who examines 
carefully their mode of occurrence. Some of them were carried up to the 
surface by liquid volcanic mud, and appear in dykes of that material like 
plums in a cake. But even there they present the same evidence of attrition. 
They assuredly have not been formed in the dykes any more than in the 
surrounding tuff. In both cases they are extraneous objects which have 
been accidentally involved in the volcanic rocks. Dr. Heddle remarks that 
the occurrence of the worn pieces of orthoclase in the tuff is an enigma to 
him. I have been as unable to frame any satisfactory explanation ot it. 
It might have been thought that within the throat ot a volcano, it in 
any circumstances, loose materials should have taken an indefinite amorphous 
aggregation. And, as has been shown in the foregoing chapters, this is 
usually the case where the materials are coarse and the vent small. Oblong 
blocks are found stuck on end, while small and large are all mixed confusedly 
Fig. 217. — Ground- plan of volcanic 
neck, Elk Harbour, showing circular 
deposition of the stratification. 
T, Tuff of the neck, the arrows showing its 
inward clip ; 15 B, Dykes ; S, Sandstones 
and shales, through which the neck has 
together. But in numerous cases where the 
tuff is more gravelly in texture, and sometimes 
even where it is coarse, traces of stratification 
may be observed. Layers of coarse and fine 
material succeed each other, as they are seen 
to do among the ordinary interstratified tuff's. 
The stratification is usually at high angles 
of inclination, often vertical. So distinctly 
do the lines of deposit appear amid the 
confused and jumbled masses, that an ob- 
server may be tempted to explain the problem 
by supposing the tuff to belong not to a 
neck, but to an interbedded deposit which 
been opened. has so mehow been broken up by dislocations. 
That the stratification, however, belongs to the original volcanic vents them- 
selves is made exceedingly clear by some of the coast-sections in the East ot 
Eife. On both sides of Elie, examples occur in which a distinct circular dis- 
position of the bedding can be traced corresponding to the general form of the 
1 Op. oil. xxviii. pp. 481 et scq., and ante, vol. i. p. 352. 
2 Occasionally the crystals can be matched in some lava-form rock of the same volcanic area , 
but many of them have no such counterparts. See vol. i. p. 62 and note. 
