202 
IDDINGS. 
pyroxene-andesite, followed by those of hornblende-pyroxene- 
andesite. These were intersected by dikes of the same 
andesites, followed by hornblende-mica-andesites and dacites, 
which closed the series. The upper portion of this volcano 
having been removed, the surface-flows of the later magmas 
are not known in this region. 
Comparison of the Eocks at Electric Peak with those 
at Sepulchre Mountain. 
Upon comparing the Tables (No. IV) representing the 
succession of eruptions at Electric Peak and at Sepulchre 
Mountain, the correspondence between the two is at once 
striking, but not more so than the correspondence between 
the rocks themselves. Leaving out of consideration the 
eruptions embraced in the divisions A, which evidently 
emanated from different sources; we observe that the series 
of* magmas that were erupted successively, and solidified 
within the strata of Electric Peak as porphyrites and 
diorites, and those that consolidated into the mass of Sepul¬ 
chre Mountain are similar; that each series commenced 
with basic magmas and closed with acidic ones. Their 
division in the table into four groups is not intended to 
convey the idea that they belong to four distinct periods of 
eruption. The whole series in each case is more correctly 
a single, irregularly interrupted succession of outbursts of 
magma that gradually changed its composition and char¬ 
acter. 
When we compare the rocks that have resulted from the 
corresponding phases of these series of eruptions, the similar¬ 
ity of the porphyritic forms is immediately recognized. The 
nature and distribution of the phenocrysts in the different 
varieties of andesite and dacite, which determine their ma- 
croscopical habit, have their exact counterparts in the differ¬ 
ent varities of porphyrites. The microscopical characters 
of the phenocrysts in the corresponding varieties of porphy¬ 
rites and of the intruded andesites and dacites are identical. 
The character of the various groundmasses, however, is differ- 
