CARBONIFEROUS VOLCANIC ROCKS OF THE FIRTH OF FORTH BASIN. 511 
granular aspect ; seldom porphyritic, but generally containing distinct blebs of 
quartz readily perceptible by the naked eye. They are all intrusive masses, 
and are confined to the necks or their vicinity. They occur as veins or dykes, 
sometimes in large neck-like masses. 
2. Microscopic Characters.—Under the microscope these rocks show a 
characteristic finely granular felsitic ground-mass, through which are scattered 
erains or irregular pieces of quartz and crystals of orthoclase (see Plate XII. 
fig. 9). One of the most interesting varieties is that already referred to as 
occurring in veins, together with basalts, in the vent on the shore to the west of 
Largo. It is exceedingly flinty in texture, and looks so like altered shale that 
I at first regarded it as such. Its extreme hardness causes it to stand out 
prominently on the beach, where its enduring surface acquires in places a kind of 
polish from the friction of sand particles across it. Seen with a low power under 
the microscope, it shows a curious reticulated structure, which, in some respects 
resembling the normal structure of perlite, is marked by numerous narrow 
bands running through the rock and often intersecting each other. These bands 
differ from the rest of the ground-mass in being clearer and less thickly granular. 
In the interspaces between them the ground-mass thickens into cloudy patches 
with traces of a fluid structure of the perlitic kind. The orthoclase crystals and 
quartz, however, are found indiscriminately in the bands and in the interspaces 
or crossing from the one to the other. The orthoclase occurs in Carlsbad twins, 
averaging from 35 to y$o of an inch in length. The quartz is not definitely 
crystallised, but has taken the form of rounded blebs, sometimes with the drop- 
like form so often to be noticed in felsites. Its granules are distinctly visible 
to the naked eye, and are crowded thickly through the rock. They abound 
in cavities. 
An interesting feature in this rock is its occurrence in one of the necks of 
the East of Fife where, with this exception, basalt is the only form of lava now 
to be seen. Throughout the great plateau of Lower Carboniferous porphyrites, 
extending from the Campsie Fells into Ayrshire, large bosses as well as veins 
of a yellow quartz-felsite are not uncommon. Yet no rock of this kind seem 
ever to have been erupted to the surface. Again, in the Lower Old Red Sand- 
stones, while the outflows of lavas are thoroughly basic porphyrites, large 
intrusions of siliceous felsites have taken place at and round the necks, but 
never at the surface. At the Pentland Hills, however, during Lower Old Red 
Sandstone times, great showers of felsitic tuff were ejected. It would appear 
that, even at volcanoes giving out basic lavas and tuff, there has frequently 
been an uprise of extremely acid lavas in and around the vents. 
