516 PROFESSOR GEIKIE ON THE 
heaver portions would be similarly ejected before the next great explosion, 
bringing up fresh streams of lava from below. 
One of the most common constituents of the tuffs is quartz, in the form of 
rounded and subangular grains. When I first observed these enclosures I 
naturally supposed them to be merely the grains of sand that might have been 
in suspension in the water or moving along the floor over which the volcanic 
detritus settled. This may be partly their origin. But I am now convinced 
that they were directly ejected from the volcanic orifices in great quantity, 
for I find them in greater or less abundance in the tuff of all the vents. They 
are singularly uniform in character, consisting of water-clear quartz, free of 
enclosures, except abundant liquid-cavities, which may often be observed in 
lines across the diameter of the quartz-grain. These particles of quartz are 
manifestly derived from the destruction of some highly silicated rock. I have 
tried to account for their presence, on the supposition that they are due to the 
thorough trituration of quartzose sandstone. But this hardly accounts for their 
complete isolation from each other, for the want of any crust such as so fre- 
quently surrounds the quartz-grains of sandstones, and for the absence of 
fragments of sandstones which had escaped disintegration, and of pieces of 
shale and such other stratified rocks as could hardly fail to be present. As I 
have just mentioned, these separate quartz-grains are sometimes found within 
the solid substance of the serpentine lapilli. They must have been enclosed in 
the original olivine-rock while it was still molten. 
Among the tuffs must be included some rocks, to which the name of volcanic 
mudstone may be applied. They are dull, dirty-green rocks, with a matrix 
varying from a fine impalpable hardened mud to a finely granular tuff, and 
containing lapilli and frequently fragments of shale, sandstone, limestone, &c. 
They occur at the margin of vents, wrapping round the projecting portions of 
the walls, and showing by wavy lines of flow distinct traces of having been in a 
pasty condition. To the east of Elie they rise through the tuff of the vents as 
dykes, which from their hardness rival basalt in their prominence above the 
surrounding softer tuff. One of these rocks is of an exceedingly close-grained 
texture, scarcely at first to be distinguished from a dull basalt, for which it has 
been mistaken. It has been already referred to as containing abundant pieces of 
a black cleavable hornblende and worn twin crystals of orthoclase. When the 
included fragments are carefully removed, their smooth surfaces leave a clean, 
sharp caston the fine-grained mudstone. Examined with the microscope, the 
rock is found to consist of a dark-brown or greenish amorphous granular matrix | 
crowded with small granules of quartz, with numerous minute lapilli of basalt 
rocks. It contains also occasional fragments of hornblende and plates of © 
biotite. 
