48 | ~SIERRA DE UMANGO 
The banded rocks in the Sierra de Umango area may be regarded 
rather as a combination of the two classes characterized above: 
flow-banding and parallel injection. In the cases, when the lateral : 
assimilation has progressed very far, i. e, when the invaded ~ 
rock to a greater extent has been dissolved in the granite, one ob- | 
serves the development of a rock banded by flow too. 
I have studied the case of lateral assimilation of amphibolite- 
schists by granite. The whole injection-banding and assimilation 
is well exposed in the Quebrada Agua de los Caballos, on the 
eastern slope of the Cerro Potrero Viejo. Here may be observed, 
Pre SN pd et ee EA eS AR 
that the amphibolite becomes by and by more acid by lateral 
mingling, not continuously, however, but in the form of bands of 
decreasing basicity. Simultaneously the amphibole changes into 
ee a a FE u u 
biotite (Plate IV, fig. 1). The last assimilation product is a biotite 
gneiss. | 
About the transformation of amphibole to biotite Van Hise! 
says: ,The change from hornblende to biotite is a much deeper 
seated alteration (compared with the alteration to chlorite), parti- 
cularly in connection with profound mechanical action“. Regar- | 
ding the change caused by granite assimilation (addition of salic — 
material) the same author states: „the feldspar........ frequently | 
furnishes the potassia, part of the aluminia and silica, while the 
amphiboles frequently furnish a part of the magnesia, aluminia 
and silica“ to form the biotite. The conditions in the Sierra de - 
Umango may offer a combination of the two possibilities, above 
cited. 
vince of Ontario, Canada. Dept. of Mines. Geol. Survey. Mem. o. Ottawa, 1910. 
P. 73. 
R. A. Daly: Geol. Reconnaissance between Golden and Kamloops, B. C., along 
the Canadian Pacific Railway. Ibidem, N:o 68. Ottawa 1915. Pages 34 and 
toll. 
C. N. Fenner: The mode of formation of certain gneisses in the highlands of — 
New Jersey. Journal of Geol. Vol. XXII. Chicago 1914. A. o. 
1 C.R. Van Hise: A Treatise on Metamorphism. Page 290. 
