
~ 
Parr VIL] _ ERUPTIVE ROCKS. : 587 
- former, would obviously be posterior in date. It will be understood 
then that the two groups have their respective limits determined 
mainly by their relations to the rocks among which they may happen 
to lie, though there are also special internal characters which help to 
discriminate them. 
_ The value of this classification for geological purposes is great. It 
enables the geologist to place and consider by themselves the granites, 
quartz-porphyries, and other crystalline masses which, though \lying 
sometimes perhaps at the roots of ancient volcanoes, and therefore 
intimately connected with volcanic action, yet owe their special 
characters to their having consolidated under pressure at some depth 
within the earth’s crust; and to arrange in another series the lavas 
and tuffs which, thrown out to the surface, bear the closest resem- 
_blance to the ejected materials from modern volcanoes. He is thus 
presented with the records of hypogene igneous action in the one 
group, and with those of superficial volcanic action in the other. He 
is furnished with a method of chronologically arranging the volcanic 
phenomena of past ages, and is thereby enabled to collect materials 
for a history of volcanic action over the globe. | 
In adopting this classification for unravelling the geological 
structure of a region where igneous rocks abound, the student will 
encounter instances where it may be difficult or impossible to decide 
in which group a particular mass of rock must be placed. He will 
bear in mind, however, that after all, such schemes of classification 
are proposed only for convenience in systematic work, and that there 
are no corresponding hard and fast lines in nature. He will recog- 
nize that all crystalline or glassy igneous rocks must be intrusive at 
a greater or less depth from the surface, for every contemporaneous 
sheet has obviously proceeded from some internal pipe or mass, so 
that though interbedded and contemporaneous with the strata at the 
top, it is intrusive in relation to the strata below. 
The characters by which an eruptive (igneous) rock may be 
distinguished are partly lithological and partly geotectonic. The 
lithological characters have already been fully given (Book II. 
p. 129). Among the more important of them are the predomi- 
nance of silicates, and notably of felspars, hornblende, mica, augite, 
olivine, &c.; a prevailing more or less thoroughly cystalline structure ; 
the frequent presence of vitreous matter, either macroscopically or mi- 
croscopically ; and the occurrence of porphyritic, cellular, pumiceous, 
slacgy, and amygdaloidal structures. These characters are never 
all united in the same rock. ‘They possess likewise various values 
as marks of eruptivity, some of them being shared with the crystal- 
line schists which were certainly not eruptive. On the whole, 
the most trustworthy lithological evidence of the eruptive character 
of a rock is the presence of glass, or traces of an original 
glassy base. We do not yet certainly know of any natural 
vitreous substance except of an eruptive nature. The occurrence or 
association of certain minerals, or varieties of minerals, in a rock 
