STRUCTURE OF ERUPTIVE ROCKS 201 
conspicuous as in the case of larger ones. It would seem more likely that 
the sudden inward dip of the rocks abutting upon a neck is due partly to 
the downward drag of the fragmental materials while they were slowly 
subsiding and becoming consolidated, and partly, perhaps, to the unequal 
yielding of the strata themselves before the pipes were filled. When a 
subaérial puy became extinct, the loose materials forming the cone would 
naturally tend to slip down into the crater and funnel, while at the same 
time the walls of the vent, exposed to the action of springs and to 
weathering generally, would also supply material—all this débris falling 
into the vent would form a steep, rudely-bedded talus having a centro- 
clinal dip. The falling away of the softer and less resistant rocks 
in the walls of the vent would tend to undermine the less yielding beds 
above them, and thus cause these to bend over. Finally, when the pipe 
had become filled, the consolidating débris, as it subsided, would drag 
down the rocks forming the walls, and thus increase their inward dip. 
In the case of a submarine puy there would be no weathering action 
upon the walls of the pipe, but it seems at least unlikely that the rocks 
should remain unaffected, and that larger and smaller portions should not 
become detached, and thus cause undermining and bending downwards 
of the rocks above. It is worthy of note, however, that this inward dip 
of the strata abutting against a neck does not invariably occur. 
Necks, like batholiths, may belong to almost any geological period. 
But inasmuch as a typical neck represents the upper portion of a pipe of 
eruption, and consequently is not of deep-seated origin, only those of 
subaqueous eruptions can date back to the earlier geological ages. Now 
and again, it is true, some Palzozoic necks seem to have erupted on 
land, but, if so, they must ere long have been submerged, for only in this 
way could these have escaped demolition. Exposed for a prolonged 
period to denudation, not only must the cones have been demolished, 
but the ancient land-surface on which these stood must have been so 
lowered that the upper portions of the pipes of eruption—the necks— 
would have been planed away, and the deeper seated roots of the old 
volcanoes laid bare. 


