78 SLATE FOBMATICXS. 



The transverse valley which descends from Piedras Negraa 

 and tho 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, steatitic, 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.ternate with 



* Magnesian limestone is of a straw-yellow colour, and contains 

 madrepores : it lies beneath red marl, or muriatiferous red sandstone. 



