fe 
\ 
| Part VII. Srort. u. § 2.] INTERSTRATIFIED TUFFS. 567 
and other strata, the lamination of which is bent down round them 
in such a way as to show that the stones fell with considerable force 
into the still soft and yielding silt or clay (Fig. 294). 
Fragmentary materials frequently occur in interstratified beds 
without any accompanying lava; this takes place much more 
commonly than do interstratified sheets of lava without beds of 
tuff, just as in recent volcanic districts 1t is more usual to find 
cones of ashes or cinders without lava than lava sheets without an 
accompaniment of ashes. Masses of fine or gravelly tuff several 
hundreds of feet in thickness, without the intervention of any lava- 
bed, may be observed in the volcanic districts of the Old Red Sand- 
stone and Carboniferous systems in Scotland, evidence of long- 
continued volcanic action, during which fragmentary materials were 
showered out and spread over the water-basins mingled with little 
or no ordinary sediment. On the other hand, in these same areas 
thin seams of tuff interlaminated with sandstone, shale, or limestone, 
afford indications of feeble intermittent volcanic explosions, whereby 
light showers of dust were discharged, which settled down quietly 
-amidst the sand, mud, or limestone accumulating around at the 
time. Under these latter circumstances tuffs often become fos- 
 siliferous; they enclose the remains of such plants and animals as 
might be lying on the lake-bottom or sea-floor over which the 
showers of volcanic dust fell, and thus they form a connecting link _ 
between aqueous and igneous rocks. 
As illustrations of the nature of the stratigraphical evidence for 


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Y ¥, d = ae 
Xx Sy a 
Fic. 295.—Srction oF INTERSTRATIFICATIONS OF TUFF AND SHALE, OLD QUARRY, 
WESTER OCHILTREE, LINLITHGOWSHIRE (LOWER CARBONIFEROUS). 
the usual carbonaceous type, with remains of terrestrial plants, lies at the 
bottom. Itis covered by a bed of nodular bluish-grey tuff (2) containing 
black shale fragments, whence we may infer that the underlying or 
some similar shale was blown out from the site of the vent that 
furnished this dust and gravel. A second black shale (3) is succeeded 
by a second thin band of fine pale yellowish tuff. Black shale (5) again 
1 See Geol. Mag. I. (1864), p. 22. 
