40 SIERRA DE UMANGO 
surface. The fact, that the rock in question appears in localities, 
where the mica schist is most intensely penetrated with granite, 3 
makes it probable, that it is a product of complete absorption of 
the named schist. — The microscope reveals the following mi- a 
neral composition: the feldspar is partly a microcline, partly 
a plagioclase of ordinary, dioritic habit. A biotite with strong pleo- 
chroism is abundant and partly transformed to a fibrous substance, 
showing a high refringence and birefringence. Its colour is trans- 
parent, light pale brownish. The mineral is evidently the sillima- 
nite-variety called fibrolite. Also muscovite is visible, almost in- 
variably associated with biotite. — The structure of the rock is 
generally evengrained without signs of kataclasis. Only the fibrolite I 
has partly a dynamic flow arrangement as shown on microphoto- 
graph (Plate III, fig. 2). The transformation of biotite to fibrolite is 
very probably a dynamic phenomenon too.! 
c) CROSSCUTTING PEGMATITIC DIKES. 
The Famatina last injection phase. 
The sharply crosscutting, broad pegmatitic dikes here are younger 
than all the mingled and molten rocks in the abyssal zone on the 
eastern side of the Vinchina valley. Doubtless they due their origin, 
however, to the Famatina injection process and form its last „acid“ 
phase, because of the fact, that the dikes show transitions to the 
ordinary aplitic granite. The crosscutting character and the conspi- 
cuously flat lying position of them indicate, however, that there 
must be a hiatus between the principal intrusion act and the erup- 
tion of these apophyses; in other words, that there was already a 
1 The principal occurrence of sillimanite is, as known, in exogene contact rocks 
and also as inclusions in a granitic magma (Cfr. Adams and Barlow, l.c. page — 
131 a. 0.). The typical development of fibrolite is, according to Rosenbusch 
(Mikr. Phys. der Min. u. Gest. Teil II. Stuttgart 1905. Page 137), in the form of a 
dynamic transformation product of biotite. In the present case no simple contact 
influence has produced the alteration because the very rock is an syntectic 
one. The dynamic process is probably of tertiary age. 
u er ae 
ER 
ae Ta PTE NEC) a eat BEE en 
aa rest ee 
A i ig nn 
te 
DE 
u 
ee ae 
ac A 
a 
é 
ig 
+ 
