chap. xiv. Andesitic and Volcanic Rocks. 479 



consists of well-crystallised white albite l (as determined 

 with the goniometer in numerous specimens both by 

 Professor Miller and myself), of less perfectly crystall- 

 ised green hornblende, often associated with much 

 mica, with chlorite and epidote, and occasionally with 

 a few grains of quartz : in one instance in Northern 

 Chile, I found crystals of orthitic or potash feldspar, 

 mingled with those of albite. Where the mica and 

 quartz are abundant, the rock cannot be distinguished 

 from granite ; and it may be called andesitic granite. 

 Where these two minerals are quite absent, and when, 

 as often then happens, the crystals of albite are imper- 

 fect and blend together, the rock may be called andes- 

 itic porphyry, which bears nearly the same relation to 

 andesitic granite that euritic porphyry does to common 

 granite. These andesitic rocks form mountain masses 

 of a white colour, which, in their general outline and 

 appearance — in their joints — in their occasionally in- 

 cluding dark-coloured, angular fragments, apparently of 

 some pre-existing rock — and in the great dikes branch- 

 ing from them into the superincumbent strata, manifest 

 a close and striking resemblance to masses of common 

 granite and syenite : I never, however, saw in these, 

 andesitic rocks, those granitic veins of segregation 

 which are so common in true granites. We have seen 

 that andesite occurs in three places in Tierra del Fuego ; 

 in Chile, from St. Fernando to Copiapo, a distance of 



1 I here, and elsewhere, call by this name, those feldspathic mine- 

 rals which cleave like albite : but it now appears (' Edin. New Phil. 

 Journ.' vol. xxiv. p. 181) that Abichhas analysed a mineral from the 

 Cordillera, associated with hornblende and quartz (probably the same 

 rock with that here under discussion), which cleaves like albite, but 

 which is a new and distinct kind, called by him Andesine. It is 

 allied to leucite, with the greater proportion of its potash replaced by 

 lime and soda. This mineral seems scarcely distinguishable from 

 albite, except by analysis. 



