CRYSTALLINE BASEMENT ' 27 
ör sign of pressure. The amphibole shows the following mi- 
- eroscopical properties: The colour is very pale greenish — grayish, 
a somewhat astonishing fact regarding the almost black aspect of 
the rock surface. The absorption is very slight, and observable only 
| in orientations parallel to the c- axis. The amphibole-prismatic-cleavage 
_is well developed. The refringence is high and also the birefringence. 
The optical character is +. According to the measurements of the 
angle of extinction, attaining 21°, the amphibole seems to be a 
variety of pargasite and the whole rock perhaps a product of metaso- 
_ matic limestone replacement. Quartz is visible only in small quantities. 
The mighty zone of „schistes lustrées* in the higher part of the 
“western side of the Sierra de Umango ridge contains, as has been 
| mentioned, a great number of concordant, regularly alternating sills 
of an amphibolite schist of some ten meters of thickness. No 
smaller ramifications or thinner layers occur besides. The appearance 
is the same as in the Asperecito Hills in the Vinchina valley, but 
granite veins are not existent here disregarding a broad granite 
zone on the very crest line of the ridge. 
_ The rock has a marked schistosity, and the schist planes are 
covered chiefly with amphibole individuals of an almost black as- 
pect. On transverse ruptures the rock seems rich in salic minerals, but 
the whole appears as a kataclastic mass. Microscopically (transverse 
| cut) one find an extremely crushed structure, formed principally from 
the breaking off of the feldspar minerals. Also the amphibole has 
_been dissolved into fragments along the lines of prismatic cleavage 
and forms subparallel lines of grains. The amphibole has quite 
_a marked absorption and seems to be of the usual kind. The felds- 
par grain-mass carries a small amount of quartz too. The size of 
the grains is variable. The larger ones consist of a plagioclase. 
The mineralogical composition of the rock seems to correspond to 
a quartz bearing diorite, once concordantly intruded into the schist 
mass. Regarding the high amount of later kataclastic metamorphose, 
see later on (page 99). 
Ss) ee > 
u. 
