432 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
concealed by herbage, but where tlie rocks have been laid bare by the sea it 
may he most instructively studied. In such shore -sections, a singular 
change of dip is often observable among the strata round the edge of a vent. 
No matter what may be the normal inclination at the locality, the beds are 
bent sharply down towards the wall of the neck, and are frequently idaced mi 
end. This structure (shown in Figs. 24, 143, 147, 148 and 149) is precisely 
the reverse of what might have been anticipated, and can hardly lie due to 
upward volcanic explosions. It is frequently associated with considerable 
metamorphism in the disturbed strata. Shales are converted into porcel- 
lanite or various jaspery rocks, according to their composition. Sandstones 
pass into quartzite, with its characteristic lustrous fracture. It is common 
to find vents surrounded with a ring of this contact-metamorphism, which, 
from the hardness and frequently vertical or highly inclined bedding of its 
strata, stands up prominently on the beach (as in Figs. 126 and 210), and 
serves to mark the jiosition of the necks from a distance. 
I have not been able to find an altogether satisfactory explanation of 
this inward dip of the strata around vents. Taking it in connection with 
the metamorphism, I am inclined to believe that it arose after the close of 
rhe long-continued volcanic action which had hardened the rocks around the 
volcanic pipe, and as the result of some kind of subsidence within the vent. 
The outpouring of so much tuff and lava as escaped from many of the vol- 
canoes would doubtless often he apt to produce cavities underneath them, and 
on the decay of volcanic energy there might lie a tendency in the solid or 
cavernous column filling up the funnel, to settle down by mere gravitation. 
So firmly, however, did much of it cohere to the sides of tlie pipe, that if it 
sank at all, it could hardly fail to drag down a portion of these sides. So 
general is this evidence of downward movement in all the volcanic districts 
of Scotland where the necks have been adequately exposed, that the 
structure may lie regarded as normal to these volcanic vents. It has been 
observed among the shore -sections of the volcanoes of the Auckland 
district. New Zealand. Mr. C. Heaphy, in an interesting, paper upon that 
district, gives a drawing of a crater and lava-stream abutting on the edge of 
a cliff where the strata bend dowrr towards the point of eruption, as in the 
numerous eases in Scotland.' 
Evidence for the jirohahle siibmrial Character of some of the Cones or Puys 
of Tuff, — From the stratigraphical data furnished by the basin of the FTrth 
of Forth, it is certain that this region, di-iring a great part of the Carbon- 
iferous period, existed as a wide shallow lagoon, sometimes overspread with 
sea-water deep enough to allow of the growth of corals, crinoids, and 
brachiopods ; at other times, shoaled to such an extent with sand and mud 
as to bo covered with wide jungles of a lepidodendroid and calamitoid 
vegetation. As volcanic action went on interruptedly during a vast section 
of that period, the vents, though generally submarine, may occasionally have 
been subierial. Indeed, we may suppose that the same vent might liegin as 
a suba(]ueous orifice and continue to eject volcanic materials, until, as these 
^ Quart, .Journ. Geol. Soc. 1860, vol. xvi. p. 21,1. 
