rd 
MODE OF EXTRUSION | 45 
the uppermost lava bed is coarse grained, without sensible marks 
of surface flow, the lowest stream in exchange reveals an amygda- 
loid structure and a very fine grain, even pumice being occasionally 
encountered. The evolution of columnar structure too, where en- 
countered, is somewhat paradoxal; on the top of the basalt pile the 
columnar development is heavy, the individuals being short and of 
important diameter, whilst the bottom flow sometimes, when ever 
they have such, exposes exellent groupings of slender pillars and 
feather-shaped radiating arrangements, which point out an indubi- 
table dependence of the actual surface of the valley bottom, which 
may consist of basalt or of palaeozoic sediments. 
The narrower parts of the valleys, where the detritus cones are 
swept away, sometimes expose the sedimentary country rock under 
the top basalt, the floor of the latter being a surface of iluviatil 
erosion, in some places with waterworn pebbles of sedimentary 
origin baked together. The sedimentary stripe is by no means the 
roof of the next basalt horizon below, as may appear from a dis- 
tance, but the contact plane, apparently almost horizontal, dips 
teally beneath the basalt flow obliquely down to the valley axis, 
thus representing the slope of the valley in a newer cycle of ero- 
sion. This fact, observed in a couple of localities, suggests, that 
the greater part of intrusive sheets (or sills), related by former ex- 
plorers, may be only apparent ones,! yet really represent extruded 
basalt flows, which follow the valley bottom at the end of 
a new cycle of erosion. River pebbles, both of sedimentary and 
basaltic origin, found somewhere at the base of this second flow and, 
where exposed, at the bottom of the following one, strengthen this 
thesis. The basaltfilled valleys expose, in their present 
Stage, an inverted sequence of basalt flows; the oldest 
flows crown the upper edges of the valleys, while the 
youngest flows cover the valley floor, apparently forming 
1 The massiveness of the sedimentary strata, without well developed bedding 
panes, is not favourable to intrusions of such regular sheets or sills. 
