CRYSTALLIZATION OP IGNEOUS ROCKS. 
103 
conduit is not a regularly shaped orifice, but a very irregular 
one, constantly changing in width and cross-section, and 
that the experience of various magmas which traverse the 
same conduit must be very different. 
(2.) Thus the later of a series of eruptions may pass 
through a conduit whose walls have been heated by the mag¬ 
mas that previously passed through it. Indeed, the latter 
end of a large body of magma will pass through hotter rocks 
than the advance end of the magma encountered. Conse¬ 
quently the rate of cooling will be different in the later and 
earlier magmas, and even in the ends of the same magma. 
It may therefore happen that the crystallization of the first 
part of a great body of magma will start earlier in the erup¬ 
tion and lower in the conduit than the last part of the same 
body, and the character of the crystallization may differ 
greatly in these parts of the magma. 
(3.) It is also probable, in the case of a large conduit per¬ 
mitting the passage of a broad body of magma, that the rate 
of cooling at the sides of the body near the walls of the con¬ 
duit will be greater than the rate of cooling at the center of 
the mass. Consequently the crystallization may commence 
at the sides before it does in the center, and the nature of the 
crystallization vary from the sides towards the center. This 
is illustrated by large intrusive bodies which have stopped 
in their conduits and which exhibit distinct differences of 
crystallization at the sides and center. The sides of such 
bodies often exhibit a porphyritic structure with medium¬ 
grained groundmass and porphyritical crystals ( phenocrysts) 
of quartz, feldspar, or other minerals, according to the rock, 
while at the center the structure is granitoid or coarse-grained, 
with no porphyritical crystals {phenocrysts). The compounds 
which crystallized from the magma at the sides of the body 
with the character of porphyritical minerals crystallized from 
the magma at the center of the body with the characters of 
allotriomorphic or granitic minerals. In some of these cases 
it is apparent that the porphyritical crystals {phenocrysts) de- 
