THE MOHltOS OF SAN JtJAK. 77 



taining veins of copper-ore.* At the foot of this mountain 

 two tine springs gush out from the serpentine. Near tiie 

 village of San Juan, the granular diabasis appears alone 

 uncovered, and takes a greenish black hue. The feldspar 

 intimately mixed with the mass, may be separated into 

 distinct crystals. The mica is very rare, and thore is no 

 quartz,. The mass assumes at the surface a yellowish crust 

 like dolerite and basalt. 



In the midst of this tract of 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 Gralera 

 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- 

 grey, shining, composed of small grains, and mixed with 

 scattered spangles of mica. This limestone yields a strong 

 effervescence 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 efferves- 

 cence 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 

 to the transition-slates, or to the kieselschiefer (schistose 

 iasper), which everywhere characterise the black transition- 

 limestones. When in fragments, they might be taken at 

 first sight for basalt or hornblende.f Another white lime- 

 stone, compact, and containing some fragments of shells, 

 backs the Morros de San Juan. I could not see the line of 

 junction of these two limestones, or that of the calcareous 

 formation and the diabasis. 



* One of these veins, on which two shafts have been sunk, wa 

 directed nor. 2'i, and dipped 80 east. The strata of the serpentine, 

 where it is stratified with some regularity, run hor. 8, and dip almost 

 perpendicularly. I found malachite disseminated in this serpentine, 

 where it passes into griinstein. 



f I had an opportunity of examining again, with the greatest care, the 

 rocks of San Juan, of Chacao, of Parapara, and of Calabozo, during my 

 stay at Mexico, where, conjointly with M. del Rio, one of the most di. 

 tinguished pupils of the school of Freyberg, I formed a geognoatical col 

 lection for the Colegio de Miaeria of New Spain. 



