PLATE IV. 
Fig. 1. Amphibolite schist, to some degree metamorphosed by the white granite. 
The amphibole shows a beginning alteration to biotite, due to the acidification of 
the rock. The specimen was taken from a transition band between amphibolite and 
granite. The pale component is plagioclase and also quartz, the darker are amphibole 
and biotite. Nic. 7. Magn. 20 X. Quebrada Agua de los Caballos, Cerro Potrero Viejo. 
Fig. 2. Dark quartzite rock from an inclusion in young granite. Pavement horn- 
fels structure. The black spots are biotite. Nic. 7. Magn. 120 X. Steep rock 
hill rising from the valley floor to the west from Nacimientos. 
