SÅ 
j ae CRYSTALLINE BASEMENT 29 
pentine, The olivine is more intensely altered, often showing 
‘only small residuals in a mass of serpentine. Except the last mi- 
neral, there are as secondary product ferri-oxyde, forming a kind of 
network in the serpentine mass and doubtless marking the former 
‘cleavage of the olivine individuals and also the limits between 
themselves. The pyroxene has partly a well developed prismatic 
cleavage. Where this cleavage lacks, it is not always easy to dis- 
tinguish the two colourless minerals from one another. 
| The ordinary amphibolite rocks in the vicinity (Las Ramaditas) 
are all very schistose. The peridotite lacks all signs of pressure 
because of the circumstance, that the rock is profoundly altered by 
serpentinization and because limestone layers in the immediate 
vicinity have formed a more plastic resistance during the diastrophic 
pressure. 
Other rocks of a similar kind are not found in the Umango „pre- 
granitic“ series. The described occurence seems therefore to be 
only a limited one. | 
3) Basic, kataclastic schists of doubtful origin. 
_ In the Umango area there are in many parts intercalations of 
„dark schists“ between the sedimentary schists differing from the 
 amphibolitic schists. Megascopically they show kataclastic secondary 
schistosity, and the original petrographical character is generally very 
difficult to decipher. All these rocks offer very excellent examples 
of kataclasis, as will be shown below. 
: Many of the types in question have a porphyritic habit. Examining 
them more carefully, one finds, that this structure is really a por- 
 phyrokataclastic one, the larger minerals are only remnants of lar- 
ger original grains, while the „groundmass“ is represented by finely 
crushed rock. The orientation is that, of dynamic flow. In some 
“cases the schistosity originated at an earlier date than the in- 
_trusion of the latest (white) granite, and the rock is therefore in 
“some degree recrystallized. 
