CHAP. Ill 
VOLCANIC CONGLOMERATES 
37 
sediments is given in Fig. 15, and is selected from a number of illustrations 
of this interesting feature which have been observed among the Lower 
Carboniferous formations of the basin (jf the Firth of Forth. A solitary 
block, imbedded in a series of strata, would not, of course, by itself afford a 
demonstration of volcanic activity. There are various ways in which such 
stones may be transported and dropped over a muddy water-bottom. They 
may, for example, be floated off attached to sea-weeds, or wrapped round 
by the roots of trees. But where a block of basalt or other volcanic rock 
has obviously descended with such force as to crush down the deposits on 
Fig. 15. — Ejected block of Basalt wliicli has fallen among Carboniferous shales and limestones, 
shore, Pettycur, Fife. 
which it fell, and when la^'as and tuff's are known to exist in the vicinity, 
there can be little hesitation in regarding such a block as having been 
ejected from a neighbouring vent, either during an explosion of exceptional 
violence or with an unusually low angle of projection. 
In conclusion, reference may conveniently be made here to another 
Variety of fragmental r'olcanic materials which cannot always be satisfac- 
torily distinguished from true tuffs, although arising from a thoroughly 
non-volcanic agency. Where a mass of lava has been exposed to denuda- 
tion, as, for instance, when a volcanic island has been formed in a lake or 
in the sea, the detritus worn away from it may be spread out like any 
other kind of sediment. Though derived from the degradation of lava, 
such a mechanical deposit is not properly a tuff, nor can it even be included 
among strictly volcanic formations. It may be called a volcanic conglomer- 
ate, rhyolitic conglomerate, diabase sandstone, felsitic shale, or by any 
other name that will adequately denote its composition and texture. But 
it may not afford proof of strictly contemporaneous volcanic activity. All 
that we are entitled to infer from such a deposit is that, at the time 
when it was laid down, volcanic rocks of a certain kind were exposed 
at the surface and were undergoing degradation. But the date of their 
original eruption may have been long previous to that of the formation of 
the detrital deposit from their waste. 
Nevertheless, it is sometimes possible to make sure that the conglomerate 
or sandstone, though wholly due to the mechanical destruction of already 
erupted lavas, wms in a general sense contemporaneous with the volcanoes 
that gave forth these lavas. The detrital formation may be traced perhaps 
up to the lavas from which it was derived, and may be found to be inter- 
