78 
ST, ATE FOEJIATICJTS. 
The transverse valley which descends from Piedras Negras 
and the village of San Juan, towards Parapara and the 
Llanos, is filled with trap-rocks, displaying close affinity 
with the formation of green slates, which they cover. Some- 
times we seem to see serpentine, sometimes griinstein, and 
sometimes dolerite and basalt. The arrangement of these 
problematical masses is not less extraordinary. Between 
San Juan, Malpaso, and Piedras Azules, they form strata 
parallel to each other; and dipping regularly northward 
at an angle of 40° or 50°, they cover even the green slates 
in concordant stratification. Lower down, towards Para- 
para and Ortiz, where the amygdaloids and phonolites are 
connected with the griinstein, everything assumes a basaltic 
aspect. Balls of griinstein heaped one upon another, form 
those rounded cones, which are found so frequently in the 
Mittelgebirge in Bohemia, near Bilin, the country of pho- 
nolites. The following is the result of my partial obser- 
vations. 
The griinstein, which at first alternated with strata of 
serpentine, or was connected with that rock by insensible 
transitions, is seen alone, sometimes in strata considerably 
inclined, and sometimes in balls with concentric strata, im- 
bedded in strata of the same substance. It lies, near Mal- 
paso, on green slates, steatitie, mingled with hornblende, 
destitute of mica and grains of quartz, dipping, like the 
griinsteins, 45° toward the north, and directed, like them, 
75° north-west. 
A great sterility prevails where these green slates predo- 
minate, no doubt on account of the magnesia they contain, 
which (as is proved by the magnesian-limestone of ^England*) 
is very hurtful to vegetation. The dip of the green slates 
continues the same ; but by degrees the direction of their 
strata becomes parallel to the general direction of the pri- 
mitive rocks of the chain of the coast. At Piedras Azules 
these slates, mingled with hornblende, cover in concordant 
stratification a blackish-blue slate, very fissile, and traversed 
by small veins of quartz. The green slates include some 
strata of griinstein, and even contain balls of that sub- 
stance. I nowhere saw the green slates a.temate with 
* Magnesian limestone is of a straw-yellow colour, and contains 
madrepores : it lies beneath red marl, or muriatiferous red sandstone. 
