

ParrVIL] = —-~-ERUPTIVE ROCKS. 585 
that originally they presented some such outline as in Fig. 267, where 
the present surface (that of Fig. 266) down to which denudation has 
7 y —S <2 
E ae f= NN : 
pe ES” = ER 7 aes Sa 
i ie Oe: 2 fe. =< 
= : ce 
oe - 
4 2 
F i E a 
= 
: : oe 
} 
> 
Fic. 267.—RESTORED OUTLINE OF THE ORIGINAL Form or GrouND IN Fic. 266 (B.). 

proceeded is represented by the dotted line n s.1 We may therefore 
a priori expect to encounter different levels of eruptivity, some rocks 
being portions of sheets that solidified at the surface, others forming 
different parts of the pipe or column that connected the superficial 
sheets with the internal reservoir whence the lava proceeded. But 
we may also infer that many masses of molten rock, after being 
driven so far upward, came to rest without ever finding their way to 
the surface. Jt cannot always be affirmed that a given mass of in- 
trusive igneous rock, now denuded and exposed at the surface, was 
ever connected with any superficial manifestation of volcanic action. 
Now there will obviously be some difference between the super- 
_ ficial and the deep-seated masses, and this difference is of so much 
importance in the interpretation of the history of volcanic action 
that it ought to be clearly kept in view.-. It would manifestly lead 
to confusion if no distinction were drawn between those igneous 
masses which reached the surface and consolidated there, like 
modern lava-streams or showers of ashes, and those which never 
found their way to the surface but consolidated at a greater or less 
depth beneath it. There must be the same division to be drawn in 
the case of every active volcano of the present day. But at a 
modern volcano only the materials which reach the surface can be 
examined, the nature and arrangement of what still lies underneath 
being matter ofinference. In the revolutions to which the crust of 
the earth has been subjected, however, denudation has, on the one 
hand, removed superficial sheets of lava and tuff, and has exposed the 
subterranean continuations of the erupted rocks; and, on the other 
hand, has laid open the very heart of masses which, though eruptive, 
seem never to have been directly connected with actual volcanic out- 
bursts. All those subterranean intruded masses, now revealed at the 
surface only after the removal of a depth of overlying rock, may be 
grouped together into one division under the names Plutonic, 
Intrusive, or Subsequent. On the other hand, all those which 
came up to the surface as ordinary volcanic rocks, whether molten 
or fragmental, and were consequently contemporaneously interstra- 
tified with the formations which happened to be in progress on the 
surface at the time, may be classed in a second group under the 
names Volcanic, Interbedded, or Contemporaneous. 
1 De la Beche, “ Geol. Observer,” p. 561, 
