44 SIERRA DE UMANGO 
and has brilliant lustering cleavage surfaces, where one may observe 
the presence of small facets. Microscopically are seen, that these 
facets find their explanation in the structure of the mineral itself. j 
The principal part of the feldspar is orthoclase in cryptoperthitic in- 2 
tergrowth with albite. In this mass there are zones of microcline q 
with transition to the cryptoperthite. These microcline zones seem q 
to be the result of a pression, identical with the „undulating extinc- 4 
tion" in the pressed quartz. This kind of secondary microcline 4 
might be called „dynamic microcline“.! The quartz component of 
the pegmatites is always very clear and shows rounded contours © 
Må 
against the feldspar. In spite of the signs of dynamic affect in the ° 
3 
feldspar no pression phenomena are observed in the quartz. 
The red granite apophyses, consisting of an evengrained aplite, 
are, as said, of a more reduced breadth. They are contemporaneous 
with the pegmatite dikes and show in parts transitions to them. The 
principal occurrences of the aplite dikes are in the mentioned zone i 
of the eastern slopes of the Umango mountains, especially in the j 
hills of Las Ramaditas. The structure of the aplite is but slightly | 
crushed. Where the dikes cut across a limestone layer, a small | 
contact zone of „skarn“, i. e., garnet and epidote, may be observed. ; 
The intruded quartzitic layers contain red garnet in smaller indi- 
viduals. 
In the crystalline rock-hills of Asperecito (to the north) are 
found pegmatitic dikes of considerable size, and also aplite. The 
pegmatite is very rich in muscovite, partly crystallized in sphaeri- 
cal aggregates, partly in the form of greater sheets, exploited in — 
past times. Turmaline (schorl) is also present here. Magnetite — 
occurs in smaller well developed individuals. | 
To the west, up to the higher mountains, the pegmatite- 
1 About the formation of secondary microcline see: A. Harker: Natural History 
of Igneous Rocks. London 1909. Pages 259—260, and J. H. L. Vogt in Tschermaks 
Min. u. Petrogr. Mitt. (2). Vol. XXIV. Pages 537—-541; an interesting application is 
to be found also in the paper of H. L. Alling: The mineralogy of the feldspars. 
Journ. of Geol. 29. 1921. p. 275. 
