CHAP. XXVII 
LAVA-STREAMS AND TUFFS OF THE PUYS 
437 
of a volcanic niiid. Hut in moat localities the discharge of tuff, though for 
a: time it may have completely obscured the^ ordinary contemporaneous 
sedimentation, was intermittent, so that in the intervals between successive 
showers of detritus, the deposition of non-volcaiiic sediment went on as 
usual. - Hence it is that hands of tufl’ whether they lie^ among lavas or 
amoncf sedimentary formations, are apt to contain iiiterstratifications of sand- 
stone,'’ shale, limestone or other detrital deposit, and to pass insensibly into 
these. The extremely gentle gradation from volcanic uito non-volcamc 
sediment, and the occasional re -appearance of thin partings of tuff bring 
vividly before the mind the slow dying out of volcanic energy among the 
Carboniferous lagoons. 
The comparatively cpiiet character of the volcanic explosions, and the 
conteinporaneous undisturbed deposition of sediment during the earlier part 
of the puy period, are exeinjililied in many sections throughout the areas 
above enumerated, as will be more fully illustrated in subseciueiit pages. 
Two typical examples may suffice for this general statement of the 
characters of the discharges of tuff in the piiy-eriiptions. In the Linlithgow- 
shire quarry represented in Tig. 150, where about ten feet of strata have 
been exposed, a black shale (1) of the usual carbonaceous character, so 
common in the Oil-shale series of this region, may be seen at the bottom of 
the section. It is covered liy a bed of nodulai 
bluish-grey tuff (2) containing black shale frag- 
ments. A second black shale (3) is succeeded 
by a second thin band of fine pale yellowish 
tuff (4). Black shale (o) again supervenes, 
containing rounded fragments of tuff, perhaps 
ejected lapilli, and passing up into a layer of 
tuff' (6). It is evident that we have liere a 
continuous deposit of black shale which was 
three times interrupted by showers of volcanic 
dust and stones. At the close of the third 
interruption, the deposition of the shale was 
renewed and continued, witli sufficient slowness 
to permit of the segregation of thin scams and^ ^ _ 
nodules of clay ironstone round the decomposing organic remains of the 
muddy bottom "(7 )• ^ fourth volcanic interlude now took place, and the 
floor of the water was mice more covered with tuff (8). But the old con- 
ditions of deposit were immediately afterwards resumed (9); the muddy 
bottom was abundantly peopled with ostracod crustaceans while many 
Ashes, whose coprolites have been left in the mud, haunted the locality. 
At last, however, a much more serious volcanic explosion took place. A 
coarse agglomeratic tuff (10), with blocks sometimes nearly three feet in 
diameter, was then thrown out, and overspread the lagoon. 
The second illustration may be taken from the admirable coast-section 
between Burntisland and Kinglmrii, where the number of intercalations of 
' See Gml. Surv. Memoir of Minburgh, p. 45. These tatfs are further described on pp. 465 et seq. 
. 150. — Section in old quarry, 
we.st of Wester Ochiltree, Lin- 
lithgowshire. Calcif'erons Sand- 
stoue series. 
