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, 

 trachytic, rhyolitic and basaltic volcanoes, according to the nature of the mineral matter 

 which each volcano 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 volcanoes and the massive eruptions of the synonymous orders of volcanic rocks. 

 The first of them is the alliance of both in regard to geographical distribution, the 

 volcanoes 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 volcanoes 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 rhyolitic volcano having been 

 active before the rhyolitic epoch, or of a basaltic volcano having originated before the 

 basaltic epoch, while geological observation goes to show that during, and immediately 

 after those epochs, the volcanoes 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 volcano 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 

 volcano, and it has to be ranked as such in our proposed classification. The enormous 

 bulk of the ancient volcano is totally built up of stratified layers of andesitic 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. At a 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. 12 The noteworthy fact illustrated by these 

 observations on Lassen's Peak, and corroborated in numerous other instances, is this : 



12 Mr. Clarence King has observed the occurrence of basalt of apparently very recent origin immediately north of 



