VOLCANIC ROCKS 103 
na although its intense pigmentation with an iron ore dust 
~ conceals to some degree the structure of the rock. The greatest 
part of the grains seems to be quartz. A clayish product occurs 
probably in the pigmented mass. This product of chemical de- 
' composition may be regarded as a result of a weathering of the 
rock under humid conditions, subsequent to the deposition of the 
: lower Gondwana layers. The tuff belongs to the first great volcanic 
_ phase during the continental period. Corresponding lava rocks are 
found farther to the northeast in a similar stratigraphical position 
- (see page 99). 
To the south of the southern end of the Cerro Villa Union there 
> follows the great upbend of the Cerro Bola with the strata ranged 
_ in the same order as in the Cerro Guandacol profile. According 
to Bodenbender it appear here many volcanic intercalations (spi- 
lites), but also tuff layers. All these dark intercalations are well 
visible on the mountain slopes to the northeast and north. 
In the syncline between the two mountains in the red sandstone 
_ beds there lie (near to Las Lajas) mighty volcanic agglomerates. 
_ These are mostly composed of angular fragments cemented by a 
dark mass of grayish colour (ash). The fragments consist of mela- 
phyre of porphyritic structure and also of quartz. Microscopically 
the melaphyre seems to be very ferriferous, having almost an opaque 
groundmass. 
From the last named locality some miles in the direction of 
Aguada de los Burros mighty series of red beds are outcropping 
| [according to Bodenbender (1911) of cretaceous (?) age] containing 
a concordant layer of a dark finegrained volcanic rock. In handspe- 
- cimen one detects small cavities. Studying the rock microscopically 
the principal part seems to be a glassy substance with numerous 
inclusions of angular quartz grains. The glass mass is very porous 
and rich in ferric oxyde, forming irregular spots. The quartz grains 
originate doubtless in the neighbouring sandstone, and the glassy 
tuff was thence evidently formed simultaneously with the sandstone. 
