448 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
not far to the eastward, but now buried under the higher parts of the 
Carboniferous Limestone, Millstone Grit and Coal-measures. Again, the 
great series of sills that gives rise to such a conspicuous range of hills in 
the north and middle of Fife may liave depended for its origin upon the 
Fig. 159. — Section showing tlie position of the basic .silk in relation to the volcanic series at 
Burntisland, Fife* 
1. Caleiforous Sandstoue aeries ; 2. Burdieliouae Liinestone ; 3. Sandstones, shales and tuffs ; 4. Basalts and tuffs, 
with intercalations of sandstone, shale and limestone ; 5. Agglomerate of the Binn of Burntisland neck ; 6. 
Basalt dyke ; 7. Dyke and sill ; S 8 S. Three sills. 
efforts of a line of vents running east and west through the centre of the 
county, but now liniied under the Coal-measures. Some vents, indeed, have 
been laid hare in that district, such as the conspicuous groups of the Saline 
Hills and the Hill of Beath, but many more may be concealed under higher 
Carboniferous strata further east. 
In the fifth place, the materials of which the sills consist link them in 
petrographical character with those that proceeded from the puys. The 
rocks of the intrusive sheets in West Lothian, Midlothian and Fife are very 
much what an examination of the bedded lavas of the pnys in the same 
region would lead us to expect. There is, of course, the marked textural 
difference between masses of molten rock which have cooled very slow!}' 
within the crust of the earth and those which have solidified with rapidity 
at the surface, the sills being for the most part much more coarsely crystal- 
line than the lavas, and more uniform in texture throughout, though 
generally finer at the margins than at the centre. There is likewise the 
further contrast arising from differences in the composition of the volcanic 
magma at widely-separated periods of its extravasation. At the time when 
the streams oi liasalt flowed out from the puys its constitution was compara- 
tively basic, in some localities even extremely basic. Any sills dating from 
that time may be expected to show an equal proportion of bases. But 
those which were injected at a long subsecpient stage in the volcanic period 
may well have been considerably more acid. 
In actual fact the petrograpliical range of the sills reasonably referable 
to the puy-eruptioiis varies from picrito or limhurgite to dolerite without 
olivine. The great majority of these slieets in the liasin of the Firth of Forth, 
where they are chiefly displayed, are dolerites (diabases), sometimes with, but 
more frequently without, olivine. They include all the more coarsely crystal- 
line rocks of the region, though occasionally they are ordinary close-grained 
basalts. I'heir texture may be observed to bear some relation to their mass, 
so far at least as that, where they occur in heels only two or three feet or 
yards in thickness, they are almost invariably closer-gTained. A cellular or 
amygdaloidal texture is seldom to he observed among them, and never where 
