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inents, they might be taken at first sight for 

 basaltes, or hornblendes *. 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. 



The transverse valley, that descends from 

 Piedras Negras and the village of San Juan to- 

 ward Parapara and the Llanos, is filled with 

 trappean rocks, displaying close relations with 

 the formation of green slates, which they cover. 

 Sometimes we seem to see serpentine, sometimes 

 gruenstein, and sometimes dolerites and basaltes. 

 The arrangement of these problematical masses 

 is not less extraordinary. Between San Juan, 

 Malpasso, and Piedras Azules, they form strata 

 parallel to each other, and dipping regularly to 

 the North at an angle of 40° or 50° ; they cover, 

 even in concordant stratification, the green slates/ 

 Lower down, toward Parapara and Ortiz, where 

 the amygdaloids and phonolites are connected 

 with the gruenstein, every thing assumes a 

 basaltic aspect. Balls of gruenstein, heaped 



* 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 I formed, 

 conjointly with Mr. del Rio, one of the most distinguished 

 pupils of the school of Freyberg, a geognostical collection for 

 the Colegio de Mineria of New Spain. 



