CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 463 
obtained small crystals from the red upper tuff of Arthur Seat, recalling in 
their general appearance those of Somma. Lumps of an augitic glass have been 
found by Dr HEDDLE, sometimes as large as a pigeon’s egg, in two of the dykes 
at Elie, and in the tuff at the Kinkell neck, near St 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. I recently found much larger rounded 
masses of a similar augitic glass, but with a distinct trace of cleavage, in a 
volcanic vent of Upper Old Red Sandstone age, at John o’ Groat’s House.* 
Biotite is not a rare mineral in some of the tuffs. It may be obtained in 
the stratified tuffs of Dunbar, in plates nearly an inch broad; but the largest 
specimen I have obtained is one from the same Elie vent which yielded the 
large felspar fragment. It measures 2}x 2x4 inches. These 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 care- 
fully their mode of occurrence. Some of them were carried up to the surface 
by liquid volcanic mud, and appear in dykes like plums ina 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 accidently involved in the 
volcanic rocks. Dr HrppLE 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 of it. 
Arrangement of Materials in Necks of Tuff and Agglomerate.—It might 
have been thought that in the throat of a volcano, if in any circumstances, loose 
materials should have taken an utterly indefinite amorphous aggregation. And 
this is usually the case where these materials are coarse and the vent small. 
Oblong blocks are found stuck on end, while small and large are all mixed 
confusedly together. But in the numerous cases where the tuff is more gravelly 
in texture traces of stratification may usually be observed. Layers of coarse 
and fine material succeed each other, as they are seen to do among the ordinary 
interstratified tuffs. 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 observer may be tempted to explain the problem 
by supposing the tuff to belong, not to a neck, but to an interbedded deposit 
which has somehow been broken up by dislocations. That the stratification, 
however, belongs to the original volcanic vents themselves, is made exceedingly 
clear by some of the coast-sections in the East of Fife. On both sides of Elie 
examples occur in which a distinct circular disposition of the bedding can be 
traced corresponding to the general form of the neck. The accompanying 
* Op. cit. xxviii. p, 481, e¢ seq. 
