442 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
1200 to 1600 feet. The lavas are varieties of basalt raugiiig in character 
from a black compact columnar to a dirty green earthy cellular or slaggy 
ixick. Each separate How may be on the average about 20 or 30 feet in 
thickness. Columnar and amorphous sheets succeed each other without any 
interposition of fragmentary material (Fig. l7l). But along the junctions of 
tire separate flows layers of red clay, like the bole between the basalts of the 
Criant’s Causeway, may frequently be noticed, 'fhe characteristic slaggy 
aspect of the upper parts of these ancient cov.Jies is sometimes remarkably 
sti'ikiuw'. The full details of this most interesting section will be given in 
later pages (p. 470). But some of its more characteristic external features 
may be understood from the views which are presented in Figs. 1 52, 153, 
170, 171. 
The general bedded character of the volcanic series is well shown in 
Fig. 153, which represents the alternations of lavas and tufls in the 
Kingswood Craig two miles to the east of Burntisland. The harder 
basalts will be seen to project as bold crags while the tuffs and other 
stratified deposits between them give rise to grassy slopes and hollows. A 
nearer' view of the alternation of lavas and tuffs with non-volcanic sedimentary 
deposits is supplied in Fig. 170, which is taken from a part of the Fife coast a 
little further to the east than the last illustration. Here one of the limestones 
of the Carboniferous Limestone series is overlain with shale and tuff, which, 
being easily disintegrated, have been cut away by the waves, leaving the 
lava above to overhang and fall off in blocks. The columnar structure of 
some of the basalts of this coast is well brought out in Fig. l7l, which 
shows further how the columns sometimes merge into an amorphous part of 
the same sheet. 
These Fife basalts illustrate admirably the peculiarities of the sheets 
of lava which are intercalated among the Carboniferous strata. They show 
how easy it generally is to discriminate between such sheets and intrusive 
sills. The tnre lavas are never so largely crystalline, nor spread out in 
such thick sheets as the sills ; they are frequently slaggy and amygdaloidal, 
especially towards the top and bottom, the central portion being generally 
more fine-grained and sometimes porphyritic. Where most highly cellular 
they often decompose into a dull, earthy, dirty-green rock. Where they form 
a thick mass they are usually composed of different beds of varying texture. 
Except the dilferences between the more compact centre and the slaggy 
layer above and below, the bedded lavas do not ju’esent any marked varia- 
tion in compositioir or structure within the same sheet. A striking exception 
to this rule, however, is furnished by the Batligate “ leckstone ” already 
described.^ This mass forms a continuation of the great basaltic ridge of 
the Bathgate Hills, and though its exact relations to the surrounding strata 
are concealed, it appears to be an interbedded and not an intrusive sheet. 
Tlie remarkable seiiai'ation of its constituent minerals into an upper, lighter 
felspathic layer, and a lower, heavier layer, rich in olivine, augite and iron- 
ores, is a structure which might be more naturally expected to occur in 
' Trans. Roy. Soc. Edin. xxix. (1879) p. 504. 
