
STRUCTURE OF ERUPTIVE ROCKS 185 
vesicular or slaggy, and contain no glass. Moreover, they 
are usually—not always—rather coarsely crystalline, and 
generally granitoid in texture. Their constituent minerals 
are often crowded with fluid-cavities, while glass- and stone- 
cavities are wanting. The Hypabyssal or less deeply seated 
rocks occasionally exhibit all these characters, but they also 
not infrequently contain sporadic areas of vesicles, and even, 
it may be, some residual glassy base or devitrified matter. 
Although often coarsely crystalline, they commonly assume 
a fine-grained and sometimes a compact texture. -Hypabyssal 
rocks thus frequently have a strong resemblance to effusive 
or lavaform rocks, from which, indeed, it is often quite 
impossible to distinguish them in mere hand-specimens. The 
contrast between these two types is consequently much less 
marked than that between plutonic rocks and true lavas. 
Even in hand-specimens a truly plutonic or abyssal rock can 
rarely or never be confounded with one which has flowed out 
at the surface and consolidated under the ordinary pressure 
of the atmosphere. 
The true character of an igneous rock, however, can only 
be satisfactorily determined by studying it in the field, and 
observing its relation to the other rock-masses amongst which 
it occurs. Usually it is not difficult to recognise an intrusive 
rock, since its junction with surrounding rock-masses is 
generally more or less irregular or discordant. Many 
observations in all parts of the world have shown that 
molten matter invading the crust from below has usually 
followed what may be considered lines of weakness. That 
crust, as we have now learned, is by no means homogeneous, 
but built up of a great variety of rocks arranged in many 
different ways, and traversed by an infinity of regular and 
irregular cracks, fissures, rents, and dislocations, many of 
which are vertical or approximately so, while others are 
inclined at all angles. All such original and superinduced 
planes of division—whether planes of bedding, cleavage, or 
foliation, whether unconformable junctions, joints, or faults— 
are lines of weakness along which molten matter has from 
time to time found more or less ready passage to the surface. 
Further, it may be noted that molten matter has not infre- 
quently made a way for itself by fusing or dissolving and 


