CHAP. XXVIII 
PUYS OF THE FIRTH OF FORTH 
467 
ot the district. The matrix of this rock is in part a dull green granular 
mudstone, wrappmg round the lapilli and ejected stones, which, when they 
thm ^ weather, leave casts of their forms behind 
them. The enclosed Iragmeiits vaiy in size up to blocks three feet in 
c idineter, and consist in great measure of a compact volcanic grit, composed of 
Lkes fragments of black shale, grains of sand and 
hakes of mica. 1 Jiere are likewise blocks of cement-stone and shale. Thin 
courjis of black shale mterlaminated with the tuff' show its beddin.v 
liie thickest and most continnoiis accumulations of tuff occur round some 
o le larger tnfl cones, particularly round the Saline Hills, and in the 
uintisland district. In the first-named area the copious eruptions of 
ragmentaiy material brought the volcanic history there to an end; but 
se^rlf prolonged and varied 
series of discharges. 
whi e the majority of the vents were tuff-cones, and emitted only fragmentary 
discliarges, there were two well-marked tracts where lavas were poured out 
xtensively and during a long geological interval. One of those lies in the 
southern, the other in the northern area. 
The southern or Linlithgowshire lava-ridge forms now what are known 
as the Bathgate and Linlithgow Hills. The lavas extend for about 14 miles 
rom north to south, dying out in both directions, while their present visible 
breadth is about three miles at its widest part. The liighest summit reaches 
a height of about 1000 feet above the sea. The structure of tliis lon« ridge 
reveals a,n interesting record of volcanic eruptions. It consists mainly tf 
sheets of basalt, sometimes separated by layers of tuff (Fio- 15 5 ) But 
on one or two horizons the volcanic rocks cease, and ordinary sedimentary 
deposits take their place. As has been already stated, the Main or Hurlet 
Limestone can be traced through the heart of the volcanic masses. This 
seam attains there an exceptional thickness of as much as 70 to 80 feet 
and IS nowhere more abundantly fossiliferous. During its deposition tliere 
seems to have been a subsidence of the area, together with a cessation of 
Dmrn " brachiopods, bryozoa, 
< elhbraiichs, gasteropods, cephalopods and fishes, which swarmed in the 
c ear water, built up a thick calcareous layer above the lavas and tufts of 
tlie sea- bottom. 
Among the sandstones and shales that cover the limestone, bands of tuff 
make their appearance, indicating the renewal of volcanic activity. These 
are immediately, surmounted by another thick pile of basalt-sheets. Subse- 
quently, during pauses in the eruptions, while the general subsidence con- 
tinued, renewed deposits of sediment spread over the submerged volcanic 
bank. One of the longest periods of quiescence was that during which the 
mukV*'T T I^i"^estone of Bathgate crept northwards over the 
sunken lavas and tuffs. But the whole of the ridge does not seem to have 
l oTbeenT 1 ^^tercalated strata have 
not been traced across the thick pile of volcanic material near Linlitlmow 
