i ; ; - 
Part VII. Secr. ii. § 1.] INTERBEDDED ROCKS. 063 
characters, but retaining the broad general aspect of a lenticular bed 
or sheet. A comparison of such a bed with one of the intrusive 
sheets already described shows that in several important lithological 
characters they differ from each other. An intrusive sheet is closest 
in grain near its upper and under surfaces. A contemporaneous bed 
or true layva-flow, on the contrary, is there usually most open and 
scoriaceous. In the one case we rarely see vesicles or amygdules, in 
the other they often abound. However rough the upper surface of 
an interbedded sheet may be, it never sends out veins into nor encloses 
portions of the superincumbent rocks, which, however, sometimes 
contain portions of it, and wrap round its hummocky irregularities. 
Occasionally it may be observed to be full of rents which have been 
filled up with sandstone or other sedimentary material. These rents 
were formed while the lava was cooling, and sand was subsequently 







































































































































































































































































































































































































Fie, 291.—SanpDsTonE (8) FILLING RENTS IN THE SURFACE OF AN INTERBEDDED SHEET 
oR FLow oF PoRPHYRITE(p). Coast oF KINCARDINESHIRE. 
The rents have been filled in with sand before the eruption of the next flow. 
washed into them. Examples of this structure abound among the 
porphyrites of the volcanic tracts of the Scottish Lower Old Red 
Sandstone. The amygdaloidal cavities throughout an interbedded 
sheet, but more especially at the top, may often be noticed with an 
elongated form, and even pulled out into tube-like hollows in one 
general direction, which was obviously the line of movement of the 
yet viscous mass (pp. 89, 477). Some kinds of rock when occurring in 
interbedded sheets are apt to assume a system of columnar jointing. 
Basait in particular is distinguished by the frequency and perfection 
of its columns. The Giant’s Causeway and the cliffs of Staffa, of 
Ardiun in Mull, of Loch Staffin in Skye, the Orgues d’Expailly 
in Auvergne, and the Kirschberg of Fulda are well-known examples 
(ante, p. 506). 
Interbedded lavas of former geological periods, like those of 
i 202 
