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564 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
recent date (ante, p. 239), occur under the two tolerably well-defined 
conditions of crater and fissure-eruptions. 
1. Single lenticular sheets or groups of sheets, usually of 
limited extent and with associated bands of tuff, form the more 
frequent type among Paleozoic and Secondary formations. <A 
single interbedded sheet may occasionally be found intercalated 
between ordinary sedimentary strata without any other volcanic — 
accompaniment. But this is unusual. In the great majority of 
cases several sheets will be found together, with accompanying 
bands of contemporaneous tuff. 
In such abundantly volcanic districts as central Scotland, the 
necks or vents of eruption (p. 558) may frequently be detected 
around the lavas which proceeded from them. ‘The thickness of an 
interbedded sheet varies for different kinds of lava. As a rule, the 
more acid rocks are in thicker beds than the more basic. Some of 
the thinnest and most persistent sheets may be observed among the 
basalts, where a thickness of not more than 12 or 15 feet for each 
sheet is not uncommon. Both individual sheets and groups of 
sheets possess a markedly lenticular character. They may be seen to 

a 
< 
Fic. 292.—Four svuccEssIvE FLtows or Porrayrire, LowER CARBONIFEROUS, Hast 
LINTON. 
thicken in a particular direction, probably that from which they 
flowed. Thus in Linlithgowshire a mass of lavas and tuffs, reaching 
a collective thickness of probably 2000 feet in the Carboniferous 
Limestone series, rapidly dies out, until within a distance of only 
ten miles it dwindles down to a single band less than fifty feet 
thick. On the other hand, beds of tolerably uniform thickness and 
flatness of surface may be found ; among the basalts, more particularly, 
the same sheet may be traceable for miles, with remarkable 
regularity of thickness and parallelism between its upper and under 
surfaces (p. 565). The porphyrites and trachytic and felsitic lavas 
are more irregular in thickness and form of surface (Fig. 292). 
Interbedded (and also intrusive) sheets have shared in all 
the subsequent curvatures and faultings of the formations among 
which they lie. This relation is well seen in the ‘ toadstones” or - 
diabase beds associated with the Carboniferous Limestone of Derby- © 
shire (Fig. 293)." 
2. The second type is displayed in widespread plateaux com- 
posed of many successive sheets, frequently with little or no inter- 
calation of tuff. It occurs even among Paleozoic formations, but 
' See Section 18, “ Hor. Sect. Geol. Surv. Great Britain.” 
