98 SIERRA DE UMANGO 
the Paso de Lamar, described already by Bodenbender (1911), | 
has clearly furnished all the material of the numerous lava inter- { 
already mentioned by Stelzner (l. c.). 
The petrographical descriptions of the volcanics irom the pampean 4 
sierras’ continental strata are not abundant. Stelzner (I.c.) says, that - 
these rocks are, when melaphyric, composed of a plagioclase and 
a purplish augite. According to this author ophitic structures are | 
gancillo there is a vast accumulation of aphanitic and spilitic Ivan 
¥ 
quite scarce. Olivine occurs sometimes, associated with picotite. — 
Magnetite is found frequently. The vesicles of the rocks, repre- — 
senting ancient superficial flows, are filled with calcite or viridite. 
Sometimes they are also empty. 
The petrographical data published by Siepert (1894) from the 
material of Brackebusch concern only to a small degree the 
Sierra de Umango area. Siepert divides the rock types into | 
„Diabases“ and „Melaphyres“. Regarding the geological age of © 
the rocks, the latter name is to be preferred. | 
Specimens of the continental volcanic rocks, collected in all parts 
of the area investigated, may offer quite a complete picture of the 
whole effusive series, corresponding to the length of the continental — 
period in these regions. Besides a description of the concordant | 
beds of superficial lavas and tuff layers here follows a such of a | 
great number ofj effusive rocks, whose age in many cases cannot - 
be exactly fixed, i. e. quartz-porphyries, lamprophyric rocks, basalts 
and some leucocratic rocks of dike appearance. A description of - 
at 3 
the 
the tertiary volcanics of some isolated occurrences will then follow. 
a) CONTINENTAL" LAVA ROCKS. 
The spilitic types are the most common among this class of rocks. - 
The dense mass is generally of a darkbrown colour, and calcite- 
or quartz-filled vesicles are dispersed therein as numberless white 
“ we 
Brg ae Sy 
