
4 A +r , 
538 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
Paty 
‘ 
may also afford presumptive evidence of its igneous origin. Sanidine, 
leucite, olivine, nepheline, for example, are for the most part charac- 
teristic voleanic minerals, and mixtures of finely crystallized triclinic 
felspars with dark augite, olivine, and magnetic iron, or with horn- 
blende, are specially met with among eruptive rocks. 
But it is the geotectonic characters on which the geologist must 
chiefly rely in establishing the eruptive nature of rocks. These 
vary according to the conditions under which the rocks have con- 
solidated. We shall consider them as they are displayed by the 
Plutonic, or deep-seated, and Volcanic, or superficial phase of 
eruptivity. 
Section I. Plutonic, Intrusive or subsequent 
Phase of Eruptivity. 
We have here to consider the structure of those eruptive masses 
which have been injected or intruded into other rocks, and have con- — 
solidated beneath the surface. One series of these masses is crystalline 
in structure, but with felsitic and vitreous varieties. It includes most 
of the eruptive rocks, and especially the older forms (granite, 
syenite, quartz-porphyry, pitchstone, diorite, &c.). The other series 
is fragmental in character, and includes the agglomerates and tuffs 
which have filled up voleanic orifices. 
After some practice, the field-geologist acquires a faculty of 
discriminating, even in hand-specimens, crystalline rocks which have 
consolidated beneath the surface from those which have flowed out 
as lava-streams. -Coarsely crystalline granites and syenites, with no 
‘trace of any vitreous ground-mass, are readily distinguishable as 
plutonic masses; while, on the other hand, cellular or slaggy lavas 
are easily recognized as superficial outflows, or as closely connected 
with them. But it will be observed that such differences of texture, 
though furnishing useful helps, are not to be regarded as always and 
in all degrees perfectly reliable. We find, for example, that some 
lavas have appeared at the surface with so coarsely crystalline a 
structure as to be readily mistaken by a casual observer for granite ; 
while, on the other hand, though an open pumiceous or slaggy 
structure is certainly indicative of a lava that has consolidated at or 
near the surface, a finely cellular character is not wholly unknown 
in intrusive sheets and dykes which have consolidated below ground. 
Again, masses of fragmentary volcanic material are justly regarded 
as proofs of the superficial manifestation of volcanism, and in the vast 
majority of cases they occur in beds which were accumulated on the 
surface as the result of successive explosions. Yet cases, which will 
be immediately described, may be found in many old voleanie districts 
where such fragmentary materials have fallen back into the voleanic 
funnels, and filling them up have been compacted there into solid 
rock, or where they may occasionally have been produced by ex- 
plosions of lava within subterranean caverns. 
The general law which has governed the intrusion of igneous 
