54 STUDIES IN GEOLOGY, No. i 



than others. The groundmass in some facies is almost 

 granitic, but is more generally microgranitic. The most 

 appropriate name for the rock would seem to be hornblende 

 diorite porphyry, although by some it has been called a 

 diorite. 



Following is Douglas' description (p. 29) of the Mirikiri 

 or Comanche rock, which he calls a diorite : 



"Macroscopic Characters : A pale-gray holocrystalline rock of 

 even-grained medium texture, composed of white plagioclase and 

 dark green hornblende, the latter frequently segregated into more 

 basic patches. No quartz visible. It often contains abundant mi- 

 caceous hematite, especially developed along the joint faces. 



"Microscopic Characters : Holocrystalline, hypidiomorphic struc- 

 ture. Plagioclase, chiefly andesine and acid labradorite, showing 

 Carlsbad and polysynthetic twining and well-marked zonary struc- 

 ture. The felspar is mostly fresh, but exhibits some alteration to 

 sericite beginning usually at the centre of the crystals. Green horn- 

 blende occurs in small well-defined crystals, with idiomorphic con- 

 tours. Ilmenite altering to sphene, and a little apatite, epidote, and 

 chlorite also are present." 



Orrego * describes the Comanche rock as intrusive in the 

 sediments, into which it has sent numerous apophyses which 

 have produced slight metamorphism along their contacts. 

 He calls the rock a diorite of pale-gray color, holocrystalline, 

 made up of andesine and probably labradorite, black horn- 

 blende and lacking in quartz, and with an abundance of 

 micaceous hematite occurring especially along joints. 



Steinmann (p. 367) says that Stelzner called the rock a 

 fresh holocrystalline, hornblende andesite, but that a speci- 

 men which he secured is fully crystalline, though markedly 

 miarolitic, and hence he regards it as a rock intermediate 

 between diorite and andesite. 



Forbes (p. 29) preferred the term diorite to andesite. He 

 says there are two belts of dioritic rocks extending through 

 the Andes in a nearly north-south line marked by discon- 



* Alfredo Escuti Orrego: Observaciones Geologicas siguiendo el 

 trazado del Ferrocarril de Arica a La Paz. Boletin de la Sociedad 

 Geografica de Lima, vol. 34, 1918, p. 82. 



