514 PROFESSOR GEIKIE ON THE 
any of them now in their original condition; so that the diffused red, brown, and 
green matter of their base may represent microlites and crystals of some 
of the constituent minerals of the lavas. 
Where the tuffs occur as beds or lamine, interstratified with sedimentary 
rocks, the paste necessarily becomes mixed with the sand, mud, or limestone 
which may have been gathering in the floor over which the volcanic eruptions 
took place. Many good specimens showing this intermixture under the micro- 
scope may be gathered from the Bathgate and Pettycur localities already 
referred to. 
b. The Lapillii—These consist chiefly of rounded or subangular fragments 
of the lavas of the district in which the tuff lies. In Fife and the Lothians 
among the districts of basalt and dolerite, fragments of these rocks may be 
detected abundantly in the tuffs. Many of them do not differ in any respect 
from the substance of the solid rock as we see it now in sheets or in dykes at the 
surface. They seem to have been derived from the breaking up of already consoli- 
dated lava. This, so far as I have been able to observe, appears to be true also of 
the whole of the lapilli generally. It is rare to meet with one which has its cells 
drawn out round its circumference in such a manner as to point to its having 
been ejected from a molten mass and having acquired its globular form from 
rapid gyration in the air during itsascent. On the other hand, every section of 
tuff will furnish examples of cellular lapilli, in which the cells have been cut 
across by the external surfaces of the fragments. These lapilli are merely 
portions of vesicular or pumiceous lava, and may have been ejected by 
explosions that disrupted the hardened frothy crust of a rock, the lower por- 
tions of which were still molten and in ebullition underneath. So extremely 
cellular are many of the lapilli, that where they fell into water they must have 
floated for some time before becoming water-logged. ‘The vesicles are filled 
with calcite, delessite, or some other product of decomposition. 
One of the most generally diffused constituents of the tuff seems to be 
peculiar to them. In its present condition it is a serpentine or serpentinous 
substance, varying in colour from a bright grass-green or celadon-green to pale 
or honey-yellow, transparent and structureless in thin slices, looking at first like — 
a green glass (see Plate XII. fig. 10). It is almost invariably cellular, some- 
times so extremely so that the vesicles form three-fourths of the mass. The 
cavities are sometimes perfectly circular, and vary from less than ;,45 to more 
than yo of an inch in diameter. More usually they are elongated, and 
occasionally have been drawn out to such an extent that they appear as 
exceedingly thin parallel lines, giving the substance a laminated aspect. In — 
some rare instances the elongation has taken place round the external parts of 
the lapilli, the inner cells remaining circular. But in almost all cases the vesicles _ 
have been broken across by the external surfaces of the lapilli. They appear 
