16 SIERRA DE UMANGO 
mingo, Alta Gracia, Mal Paso, etc.) are very poor in MgO.! The | | 
Cördoba limestones are considered of lower silurian age.? 
The limestones of the Precordillera, i. e. of silurian age too, but ‘ 
unmetamorphosed, from Salagasta, province of Mendoza, show very 5 
small amounts of MgO. 
Microscopically the Umango limestone shows a structure of isometric 
grains, with quite angulated contours. The characteristic twinning- 
lamellae after — 1/2 R are shown elsewhere. Accessoric minerals are met = 
with only at a small extent and consist principally of quartz and 
sericite. A dominant direction in the structure is lacking and also 
every sign of kataclasis. 
In parts where the limestone occurs only as thin interstrati- 
fications in quartzitic schists etc., it has a more grayish-bluish 
colour and shows almost a banded transition to the neighbouring 
rock. 
A common feature is a banded intercalation of basic effusive layers 
(amphibolitic schists) in the limestone, a peculiarity treated off later on. 
The quartzitic schists occur, as stated above, partly as broader, 
partly as thinner zones or interstratifications. In the first case more 
schistose types of grey and dark colours are met with on the 
- eastern slope of the Sierra de Umango ridge. In the second case 
there are milkwhite, glassy layers or more concordant veins. 
The quartzitic schists have the schistosity planes marked by mica 
(biotite), often leading to quartzitic mica schists. The milkwhite 
quartz veins of layer-appearance occur principally in mica schists, 
where there are also thin layers of limestone. The intimate alter- 
nance between these layers may produce a marked banding struc- 
ture on the rock surfaces (comp. later on). 
1 A. Cogliati: Consideraciones sobre algunos calcareos y cementos de la 
Republica Argentina. Buenos Aires. 
2 H. Keidel (1914), page 674, and R. Beder: Las cales cristalino-granulosas 
de la Sierra de Cordoba y sus fendmenos de contacto. Direcciön G:ral de Minas, 
Geol. e Hidrolog. Boletin num. 7. Serie B. Buenos Aires 1914. 
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