the Rocks of the Transandine Tunnel. 43 



lime-soda felspars — oligoclase predominant — in a ground- 

 mass of oligoclase microliths. Magnetite in small grains 

 dusted over the field. No glass, but areas stained green and 

 cryptocrystalline- Colour grey. 



An Oligoclase-Trachyte, somewhat decomposed. 



Specimen 12. — Phenocrysts of tabular and columnar potash 

 and soda lime felspars, in a ground-mass in which small 

 oligoclase felspars predominate over crystals of soda-lime 

 felspar. Some augite crystals. No glass. Much iron oxide 

 in grains, and irregular areas. Colour red-brown. 



An altered Trachyte. 



Specimen 13. — A coarse-grained, dark brown rock, partially 

 disintegrated. Consists of irregular grains of different lavas 

 cemented in a nearly opaque matrix, much clouded by iron 

 oxide. A few grains of olivine. 



An altered, basic Tuff. 



Specimen 14. — A few phenocrysts of columnar lime-soda 

 felspars, and sanidine, in a ground- mass consisting of oligo- 

 clase microliths with much calcite in large areas and irregular 

 cracks. No glass; but much green chloritic matter de- 

 veloped — possibly altered glass. Some grains of magnetite. 

 Colour medium grey. Grain fine. 



A partially decomposed Sanidine-Oligoclase-Trachyte. 



Specimen 17. — Consists of andesitic fragments with calcific 

 and ferruginous alteration products in a felspathic ground- 

 mass. All constituents in various stages of decomposition. 

 Colour light. Grain medium. 



A decomposed Felspathic Tuff. 



Specimen 18. — Consists of columnar phenocrysts of sanidine 

 and oligoclase, with more basic felspars, in a ground-mass of 

 oligoclase microliths and calcareous matter. Much chloritic 

 and calcific matter developed throughout. No magnetite. 

 No glass. Colour light grey. 



A Sanidine-Oligoclase-Trachyte, partly decomposed. 



It was not considered necessary to examine microscopically 

 the remainder of the specimens, which have been named by 

 comparison with those selected. 



The following table shows the radioactivity of the speci- 

 mens determined. The thorium content was estimated from 

 the same solutions by Professor Joly, using the method 

 described by him (Phil. Mag. May & July, 1909). 



