70 ; SIERRA DE UMANGO 
greenish matrix is here quite scarce. This conglomerate is well visible | 
in a locality called Arroyo Blanquito. The dip of the conglomeratic | 
layer is here very steep, nearly vertical, but this fact is due to a ~ 
local faulting of a later age than the general tilting. Upward the 
conglomerate changes its lithological habit thus that the size of the 
boulders decreases, while the relative quantity of the matrix is 
growing mightier. The forms of the boulders change upwards to ~ 
more rounded ones. Finally the matrix is almost dominant, and — 
a ,graywacke“ results without any conspicuous bedding. The rock 
contains small irregularly scattered boulders, and is quite well ce- 
mented: a polyhedrical jointing may be observed. 
There is no doubt about the genesis of the ,graywacke“ rock, 
observed already by Bodenbender in another locality (see later 
on). The rock reminds one of the permian glacial boulder beds in 
the Precordilleran zone farther to the south, discovered by Keidel 
(1914). The lithological resemblance of the tillite described by 
Keidel in the classical profile at Jachal (I had also an opportunity 
of seeing the rock) is very great. The coarse conglomerate at the — 
base of the „graywacke“ in Arroyo Blanquito is clearly not water- 
worn. Although striae are not yet discovered, neither on the boul- 
ders nor on the basal plane, the conglomerate may be considered 
a moraine boulder bed. 
Upward from the „graywacke“ in the profile (to the south) there fol- 
lows a sandstone of grayish colour, nearer the base alternating with a 
clayish slate. These rocks are overlaid with a similar slate as below 
and of considerable thickness. The slate is covered with a grayish 
sandstone. 
I am inclined to regard the pale-coloured sandstones and slates — 
near to the roof of the graywacke as a pelodite! or glacifluvial out- 
wash products from a time subsequent to the deposition of the 
1 J.E. Woodworth: Geological Expedition to Brazil and Chile 1908—1909. 
Bull. of the Museum of Comp. Zool. Harvard Univ. Vol. LVI. N:o 1. Cambridge, 
Mass. 1912. Page 78. 
+ SENSE en ge eee AA 
