216 
IDDINGS. 
Y. The various portions of the magmas solidified under a 
variety of physical conditions, imposed by the differ¬ 
ent geological environment of each, the most strongly 
contrasted of which were the rapid cooling of the 
surface flows under very slight pressure, and the ex¬ 
tremely slow cooling of the magmas remaining with¬ 
in the conduit under somewhat greater pressure. 
It is to be remarked that the first of a series of erup¬ 
tions would pass through a colder conduit than the 
last magmas Would, consequently the rate of cooling 
would be different in each case, and the pressures 
at which crystallization sets in would also differ. 
This difference would also obtain between the 
advance and rear ends of the magma of a single 
eruption, and should be more marked in the first 
eruptions of a series than in the last, since the tem¬ 
perature of the conduit increases during the period of 
eruptions. Hence the first part of a magma may 
carry porphyritical crystals while the latter portion 
may be free from them. 
VI. We have in this region the remnant of a volcano, which 
has been fractured across its conduit, has been faulted 
and considerably eroded; and which presents for 
investigation, on the one hand, the lower portion of 
its accumulated debris of lavas, with a part of the 
upper end of the conduit filled with the final intru¬ 
sions ; and on the other hand a section of the conduit 
within the sedimentary strata upon which the vol¬ 
cano has been built. 
We find that the lower portion of the basic breccia 
is made up of andesites carrying phenocrysts of 
augite, hypersthene and plagioclase, and of other basic 
andesites without macroscopic crystals. The former 
correspond to a facies of the diorite in the conduit 
carrying the same kinds of phenocrysts; while the 
second modification of the andesite corresponds to the 
greater part of the basic diorite. For it is evident 
