279 



village of St. Juan, the granular diabasis appears 

 alone uncovered, and takes a greenish black hue. 

 The feld-spar, intimately mixed with the mass, 

 may be separated into distinct crystals. The 

 mica is very rare, and there is no quartz. The 

 mass assumes at the surfact a yellowish crust, 

 like dolerite and basaltes. 



In the midst of this tract ol trap-formation, 

 the Morros of San Juan rise like two castles in 

 ruins. They appear linked to the mornes of St. 

 Sebastian, and to La Galera, which bounds the 

 Llanos like a rocky wall. The Morros of San 

 Juan are formed of limestone of a crystalline 

 texture ; sometimes very compact, sometimes 

 spongy, of a greenish gray, shining, composed of 

 small grains, and mixed with scattered spangles 

 of mica. This limestone yields a strong effer- 

 vescence with acids. I could not find in it any 

 vestige of organized bodies. It contains, in 

 subordinate strata, masses of hardened clay, of 

 a blackish blue, and carburetted. These masses 

 are fissile, very heavy, and loaded with iron ; 

 their streak is whitish, and they produce no 

 effervescence with acids. They assume at their 

 surface, by their decomposition in the air, a 

 yellow colour. We seem to recognize in these 

 argillaceous strata a tendency either toward the 

 transition-slates, or toward the kieselschiefer 

 (schistoid jasper) which everywhere characterize 

 the black transition limestones. When in frag- 



