CEREO DE FLORES. 79 



the black elates of the ravine of Piedras Azules : at tts 

 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. 



Farther 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, w T hich farther north was less granulous, 

 and passed into serpentine, here assumes a very different 

 character. It contains balls of mandelstein, or amygdaloid, 

 eight 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 

 as basalt, and 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 be 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 

 it 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 grunstein of ten or 

 fourteen inches thick ; sometimes (and this situation is most 

 common) the balls of mandelstein, two or three feet in 

 Hituneter, are found in 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. 



South-west of the village of Parapara rises the little Cerro 

 de Flores, which is discerned from afar in the steppes 

 Almost at its foot, and in the midst of the mandelstein 

 tract we have just been describing, a porphyritic phonolite, 

 A mass of compact feldspar of a greenish grey, or mountain- 



