Revieivs — Richthoferis System of Volcanic Rocks. f»ll 



stratified layers of Andesitic tufa and lapilli, which, in the steep 

 gorge issuing from its lower crater, are exposed in a thickness of 

 nearly four thousand feet Besides these stupendous accumula- 

 tions 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 

 north-eastern part of the wall of the crater; its currents have 

 expanded to eloyigated 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." 



If these "volcanic rocks," are not worthy of the epithet 

 " massive," we may, at least, inquire how they are distinguishable 

 from those massive rocks, of Avhich M. Eichthofen contrasts the 

 " Grandeur " (p. 90) with the " insignificant processes of volcanic 

 activity," and the same question may be asked in reference to all 

 those hundreds of volcanic peaks and cones with which the Andes 

 of both continents are studded, and which rise from 5,000 to 25,000 

 feet above their bases — all the acknowledged products of this 

 " insignificant volcanic activity." The following is the only passage 

 we can find in the memoir, in which any attempt is made to justify 

 the two-fold division of " volcanic rocks." After admitting that 

 "no distinct line of demarcation can be drawn between the two 

 modes of manifestation of subterranean energy," he goes on to say, 

 "But in numerous instances . . . conspicuous differences may be 

 noted between them. Volcanos are provided loith a channel connecting 

 the seat of volcanic action with the surface. The matter which they 

 eject consists either of stratified layers of ashes and scoria3, or of 

 currents of lava in the shape of flat sheets, or of alternating layers 

 of both kinds of material, &c." " Similar rocks when theij come to 

 the surface by 'massive eruptions,' do not present these distinguishing 

 features. They usually compose ranges of small width in pro- 

 portion to their length, and in place of one or more distinct centres, 

 an elongated axis may be detected." They present " a certain massive 

 character." " 'Masses' are frequently found thousands of feet in 

 height, which do not vary perceptibly in character, and show no 

 horizontal structure." The " breccias," which sometimes accompany 

 them in very large masses, " do not often occur in stratified laj^ers." 

 "The ranges show no signs of craters — yet they are frequently the 

 foundations of Volcanos. Oftener still do Volcanos occur on the lower 

 portions of their slopes, or in a series parallel to the axis of the main 

 range, and even greatly exceed it in elevation. Notwithstanding 

 these points of difference, it is often difficult to decide Avhat was the 

 mode of origin of an accumulation of volcanic rocks " (p. 61). 



Well may this be, since the points of difference are certainly quite 

 undistinguishable. It is evident that we have here the old error of 



