APFENDIX—NOTE ON ROCKS FROM THE ANDES. 141 



i)f lime and .soda, occupying thus a position intermediate l)etween oligoclase 

 (which has distinctly more soda than lime), and labradorite (whicli has more 

 lime than soda). The silica percentage also is intermediate between those of 

 the same two felspars. Whetlier there be snch a felsjsar, capable of distinc- 

 tion as a species, we mnst leave to mineralogists to decide ; and to settle how 

 many are the true species of felsjjar is a c[uestion not ea.sy to solve. The 

 rock Andesite, however, is a well-recognised and fairly definite one. The 

 dominant felsjiar is always plagioclastic, and commonly, if felspar be present 

 both in distinct crystals and in microlitlis, the former contain the lower per- 

 centage of silica and approximate to lal)radorite, the latter contain the higher 

 and come nearer to oligoclase. Cliemically the rock is di.stinguished liy the 

 predominance of soda among its alkaline base.s. The jiercentage of silica may 

 fall rather lielow 60, in the more liasic A^arieties, or ri.se up to more than 

 70 in the most acid varieties (often called Dacites) in which fiee quartz 

 is usually present. In the c^uartzless grouj), however, the silica iJercentage 

 is geneially not more than 65.i Lithologically the rock consists of a glassy 

 l>ase (often crowded with micrcjlithic products) in which are usually scattered 

 larger cry.stals of felspar, of a pyroxenic mineral, and of iron -oxide. The 

 l^yroxenic constituent may be one or more of the following : — augite, horn- 

 blende, hypersthene ; ^ occasionally there is a little biotite. 



The great majority of the specimens collected Ijy ]\Ir. Whymper lielong to 

 the c[uartzless division of the Andesites, only an outlying part of one moun- 

 tain (Antisana) having furni.shed varieties which have a silica percentage 

 exceeding 70. The latter are pitchstones of ordinary aspect, being probably 

 very vitreous members of the Dacite group. The most basic forms, repre- 

 sented by the nearly black, .suljvitreous rock, which Ijut for a i>eculiar resin- 

 ous aspect and rather too low specific gra\'ity might readily be classed as 

 a basalt (the melaphyre and the pitchstone porphyrite of some authors), also 

 seem to be rare ; the majority of the specimens varying in colour from light 

 greyi.sh or yellowish or reddish to duller tints of grey or red, which in onlv 

 a few cases api:)roach black. 



I give below a taljle of the volcanic products of each mountain, enumer- 

 ating them from north to south.-'^ 



1 Numerous analyses have Leen published. Several will lie found in Zirkel, 

 Microscopical Petrology (Report of the Geol. Expl. of the 40tli Parallel, U.S.A.). 



2 It has been doubted whether this be only a dimorphic (rhombic) foi m of augite, or 

 true hypersthene, but recent investigations show that its analysis warrants the reference 

 to the latter mineral. 



^ The following explanation may be useful. If the observed pyro.xeuic constituent 

 is wholly, or almost wholly, augite, the rock is called an augite-andesite ; if hornblende, 

 a honiblendc-andesitc ; and when there is a marked amount of hypersthene the epithet 

 hypcrstheniferous is added, and so on. 



