part 1] THROUGH THE ANDES OF PERU AND BOLIVIA. 51 



rectangular cross-sections, and a grey anorthoclase with characteristic rhomb- 

 shaped outlines. The two minerals are not easily distinguished one from 

 the other, except for the cleavage-faces of the felspar. The ground-mass 

 is macrocrystalline, and usually black or dark grey. Where it has undergone 

 crushing, however, it frequently assumes a red or green tint. 



Microscopic characters. — Large porphyritic crystals of felspar and 

 elseolite are present in about equal proportions. The former (anorthoclase)- 

 showing microperthitic intergrowths, are slightly cloudy through decompo- 

 sition, and include numerous prismatic crystals of ekeolite. The larger 

 phenocrysts of elaaolite are irregularly cracked, and though glassy-clear in 

 ordinary light, are seen, between crossed nicols, to have undergone considerable 

 alteration ; resulting in confused flaky aggregates of brilliantly-polarizing 

 -colourless mica (probably gieseckite). 



Smaller crystals of a pale-green augite are present in some abundance, and 

 more sparingly olivine in rounded grains with dense magnetite-rims. Apatite 

 ■occurs as inclusions in the augite. 



The holocrystalline fine-grained ground-mass is composed of minute prisms 

 ■of augite, felspar-laths with straight extinction, abundant grains of magnetite, 

 and a second generation of elseolite, the low double refraction of which causes 

 it at first sight to appear isotropic. 



In the crushed and banded rock the structure of the ground-mass has been 

 almost obliterated, while porphyritic crystals remain for a time as uncrushed 

 ' eyes ' in the mylonitic matrix. With further crushing these also become 

 ground down and drawn out into elongated lenticles. 



The felspar-crystals show all the phenomena of mechanical deformation, the 

 ■cracks in many cases being filled up with secondary growths of felspar sub- 

 stance. The original inclusions of elasolite have been converted into a colour- 

 less isotropic mineral, presenting the appearance of sodalite. and the same 

 mineral has been produced (together with flaky aggregates of white mica) at 

 "the expense of the larger phenocrysts. 



Lenticular patches composed entirely of serpentine and calcite represent 

 ■original crystals of pyroxene. 



A fine-grained quartz-felspar mosaic is characteristic of the final stages of 

 crushing. 



A 123. Elseolite - syenite. Ollachea. 



A pinkish-grey holocrystalline rock of granitoid texture ; containing ortho- 

 clase (pink), ekeolite (dull white), sodalite (bluish white), and segirine (black). 



Microscopic characters. — The bulk of the rock is made up of alkali- 

 felspars (orthoclase and perthite), and elasolite, to which the coloured minerals 

 .are subordinate. 



The elasolite shows definite crystal outlines and is idiomorphic to the 

 felspar ; it has been almost completely altered into dense scaly aggregates of 

 zeolitic material (spreustein). 



Most of the felspar is in the form of perthite ; it shows evidence of having 

 been considerably crushed and fractured, the cracks in many cases being 

 filled with secondary albite showing twin-lamella?. 



Sodalite occurs in some abundance, in irregular patches or veins. 



The coloured minerals, which have crystallized after the felspar, consist of 

 deep-green asgirine and brown acmite. 



Accessory minerals are ilmenite altering to leucoxene ; numerous prisms of 

 apatite ; and occasional crystals of zircon. 



A 127. Augite -syenite. Ollachea. (PI. II, fig. 6.) 



A holocrystalline granitoid rock of medium grain, having a mottled black- 

 and-white appearance, owing to the segregation of the coloured minerals into 

 patches. 



Microscopic characters. — The association of minerals is that com- 

 monly met with in syenites of the laurvikite type and the alkali-gabbros. 



The felspars and coloured minerals are present in about equal proportions, 



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