536 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. — 
a 
Every truly volcanic mass which, by being poured out as a lava- 
stream at the surface, came to be regularly interstratified with con- 
temporaneous accumulations, must have been directly connected 
below with molten matter which did not reach the surface. One 
a, 
x 
. 


: 
It is obvious that these can be used only as relative terms. — 
part of the total mass therefore would be included in the second ~ 
group, while another portion, if ever exposed by geological revolu- 
tions, would be classed with the first group. Seldom, however, can 
the same masses which flowed out at the surface be traced directly 
to their original underground prolongations. : 
It is evident that an intrusive rock, though necessarily subse- 
quent in age to the rocks through which it has been thrust, need not 
be long subsequent. Its relative date can only be certainly 
affirmed with reference to the rocks through which it has broken. 
It must obviously be younger than these, even though they he upon 
it, if they bear evidence of alteration by its influence. The probable 
geological date of its eruption must be decided by evidence to be 
obtained from the grouping of the rocks all around. Its intrusive 
character can only certainly determine the limit of its antiquity. 
We know that it must be younger than the rocks it has invaded; 
how much younger must be otherwise determined. Thus, a mass of 
granite or a series of granite veins (a a, Fig. 268) is manifestly 

Fic. 268.—SEcTION SHOWING THE RELATIVE AGE or AN INTRUSIVE Rock (B.). 
osterior in date to the rocks (b 6) through which it has risen. 
But it must be regarded as older than overlying undisturbed and 
unaltered rocks (c), or than others lying at some distance (ef) which 
contain worn fragments derived from the granite. 
On the other hand, an interbedded or contemporaneous igneous 
rock has its date precisely fixed by the geological horizon on which 
it lies. Sheets of lava or tuff interposed between strata in which 
such fossils as Calymene Blumenbachu, Leptena sericea, Atrypa reti- 
cularis, Orthis elegantula, and Pentamerus Knightiz occur, would 
be unhesitatingly assigned by a geologist to submarine volcanic 
eruptions of Upper Silurian age. A lava-bed or tuff interealated 
among strata containing Sphenopteris affinis, Lepidodendron Velthei- 
mianwm, Leperditia, and other associated fossils, would unequivocally 
yrove the existence of volcanic action at the surface during the 
ice Carboniferous period, and at that particular part of the 
period represented by the horizon of the volcanic bed. Similar 
eruptive material associated with Ammonites, Belemnites, Pentacrinites, 
&c., would certainly belong to some zone in the great Mesozoic suite 
of formations. An interbedded and an intrusive mass found on the 
same platform of strata need not necessarily be coeval. On the 
contrary, the latter, if clearly intruded along the horizon of the 
