CERRO DE FLORES. 
79 
the black slates of the ravine of Piedras Azules : at ths June 
of junction these two slates appear rather to pass one into 
the other, the green slates becoming of a pearl-grey in pro- 
portion as they lose their hornblende. 
Parther south, towards Parapara and Ortiz, the slates dis- 
appear. They are concealed under a trap-formation more 
varied in its aspect. The soil becomes more fertile ; the 
rocky masses alternate with strata of clay, which appear to 
be produced by the decomposition of the griinsteins, the 
amygdaloids, and the phonolites. 
The griinstein, which farther north was less granulous, 
and passed into serpentine, here assumes a very different 
character. It contains balls of mandelstein, or amygdaloid, 
e’ght or ten inches in diameter. These balls, sometimes a 
little flattened, are divided into concentric layers : this is 
the effect of decomposition. Their nucleus is almost as hard 
f s aud they are intermingled with little cavities, owing 
to bubbles of gas, filled with green earth, and crystals of 
pyroxene and mesotype. Their basis is greyish blue, rather 
soft, and showing small white spots which, by the regular 
form they present, I should conceive to he decomposed feld- 
spar. M. von Buch examined with a powerful lens the 
species we brought. . He discovered that each crystal of 
pyroxene, enveloped in the earthy mass, is separated from 
't by fissures parallel to the sides of the crystal. These 
fissures seem to be the effect of a contraction which the 
mass or basis of the mandelstein has undergone. I some- 
times saw these balls of mandelstein arranged in strata and 
separated from each other by beds of griinstein of ten or 
fourteen rnches thick ; sometimes (and this situation is most 
common) the balls of mandelstein, two or three feet in 
uiameter, are found m heaps, and form little mounts with 
rounded summits, like spheroidal basalt. The clay which 
separates^ these amygdaloid concretions arises from the de- 
composition of their crust. They acquire by the contact of 
the air a very thin coating of yellow ochre. 
houth-west of the village of Parapara rises the little Cerro 
e lores, which is discerned from afar in the steppes 
Aunost at its foot, and in the midst of the mandelstein 
rract we have just been describing, a porphyritie phonolite, 
a mass of compact feldspar of a greenish grey, or mountain- 
