OF VOLCANIC ROCKS. 33 
grander vents it has undergone a periodical change. Volcanoes, whether active or 
extinct, may be classified from this point of view. We shall distinguish: andesitic, 
trachytie, rhyolitie and basaltic volcanoes, according to the nature of the mineral matter 
which each voleano has ejected in the first epoch of its activity, regardless of any later 
changes. Two noteworthy relations may be traced between these different orders 
of voleanoes and the massive eruptions of the synonymous orders of voleanie rocks. 
The first of them is the alliance of both in regard to geographical distribution, the 
voleanoes of each order being limited, in this respect, to the immediate neighborhood 
of massive accumulations of rocks similar in nature to their first lavas. From this 
may partly be inferred the second relation, that the massive eruptions of each order 
have been succeeded by volcanic activity, which occasioned the ejection of lava corre- 
sponding in nature to their own rocks, and continued for long after-time, in many instan- 
ces to the present day. This dependence of volcanoes upon massive eruptions explains 
why the number of active voleanoes is so small when compared with those which 
are extinct, and why the present activity even of those which are still in operation, ap- 
pears to be only a faint remnant of that which the same vents exhibited in former time. 
It will further explain why no vestige can be found of a rhyolitie voleano having been 
active before the rhyolitic epoch, or of a basaltic voleano having originated before the 
basaltic epoch, while geological observation goes to show that during, and immediately 
after those epochs, the voleanoes of either order have been most intense, numerous 
and extensive, and their activity has, from that epoch of culmination, gradually relaxed, 
in most cases to perfect extinction. 
An instructive instance of one of those grander volcanoes which have undergone 
a periodical change in regard to the nature of the matter ejected from them, is afforded 
by the extinct voleano Lassen’s Peak, in Northern California, which Professor J.D. 
Whitney and I visited in 1866. We found it to have been originally an andesitic 
voleano, and it has to be ranked as such in our proposed classification. The enormous 
bulk of the ancient voleano is totally built up of stratified layers of andesitie tufa 
and rapilli, which, in the steep gorge issuing from its lower crater, are exposed in a 
thickness of nearly four thousand feet, notwithstanding the total destruction which 
the upper part of the former cone has undergone, and the fact of its lower parts 
extending down far beneath the present surface, and being therefore concealed to view. 
Besides these stupendous accumulations of loose matter, currents of andesitic lava 
appear to have been emitted from the crater, extending at least twenty miles from 
the place of ejection. Ata later epoch, the activity of the same volcano has been 
distinguished by the emission of trachytic lava from the northeastern part of the 
wall of the crater; its currents have expanded to elongated and sloping tables. 
bounded by abrupt descents. A third epoch is marked by the outbreak of rhyolite 
at the same place whence the trachytic rocks had issued. Rhyolite composes the 
present summit of Lassen’s Peak, on which it is accumulated in a thickness of more 
than fifteen hundred feet, also some other summits of less altitude, and at least one 
prominent current of lava of great volume.” The noteworthy fact illustrated by these 
observations on Lassen’s Peak, and corroborated in numerous other instances, is this : 
12 Mr. Gineree Kine has preeed the occurrence of cae ult of Poe very recent origin Metal) nari of 
(71) 
