62 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
agglomerates of Mull and Skye will be described in subsequent pages, 
likewise the intercalation of rhyolitic detritus between the basalts of 
Antrim. A similar association occurs among the modern vents of Iceland. 
Among the contents of the tuffs and agglomerates that occupy old 
volcanic’ vents, some are occasionally to he observed of which the source is 
not easily conjectured. Detached crystals of various minerals sometimes 
occur abundantly which were certainly not formed hi situ, but must have 
been ejected as loose lapilli with the other volcanic detritus. Where these 
crystals belong to minerals that enter into the composition of the lavas of 
the district in which they are found, they may be regarded as having 
probably been derived from the explosion of such lavas in the vents, the 
molten magma being blown into dust, and its already formed crystals 
being liberated and expelled as separate grains. But it seems to be 
extremely rare to find any neighbouring lava in which the minerals in' 
question are so largely and so perfectly crystallized as they are in these 
loose crystals of the neck. The beautifully complete crystals of augite 
found in the old tuffs of Vesuvius and on the flanks of Stromboli may 
be paralleled among Palaeozoic tuffs and agglomerates in Britain. Thus the 
necks belonging to the Areiiig and Llandeilo volcanoes of southern Scotland 
are sometimes crowded with augite, varying from minute seed-lilce grains 
up to perfectly formed crystals as large as hazel nuts. Tire conditions 
under which such well -shaped idiom orphic minerals were formed were 
probably different from those that governed the cooling and consolidation 
of the ordiirary lavas. 
But besides the minerals that may be claimed as belonging to the 
volcanic series of a district, others occur not infrequently in some tuff-necks, 
the origin of which is extremely puzzling. Such are the large felspars, 
micas, garnets and the various gems that have been obtained fronr necks. 
The large size of some of these crystals and their frequently perfect crystallo- 
graphic forms negative the idea that they can, as a rule, be dei'i\'ed from 
the destruction of any known rocks, though they may sometimes l)e con- 
ceivably the residue left after the solution of the other constituents of a 
rock by the underground magma, like the large residual felspars enclosed in 
some dykes. The crystals in question, however, seem rather to point to 
some chemical processes still unknown, wliich, in the depths of a volcanic 
focus, under conditions of pressure and temperature which we may speculate 
about but can perhaps hardly ever imitate in our laboratories, lead to the 
elaboration of the diamond, garnet, sahlite, smaragdite, zircon and other 
minerals.^ Examples of such foreign or deep-seated crystals will be 
described from the probably Permian necks of Central Scotland. 
Whatsoever may be the soin'ce and nature of the fragmentary materials 
that fill old volcanic vents, they present, as a gcnei'al rule, no definite arrange- 
ment in the necks. Blocks of all sizes are scattered promiscuously through 
■ t’ov lists of the minerals found in the diamond-bearing necks of Kiniherloy, see M. Bontan in 
Freniy’s Ewydupidk Chimiqve (18iS6), vol. ii. p. 168 ; Ur. M. Bauer’s EdclMcinkiinde. (1895), 
p. 223. 
