THE MOHCOS OF SAN JUAN. 387 



quartz; its mass is formed of small crystals of felspar, 

 intermixed with crystals of amphibole. This rock of diorite 

 is covered at its surface, by the effect of decomposition, 

 \vith a yellowish crust, like that of basalts and dolerites. 

 Serpentine, of a dull olive-green and smooth fracture, mixed 

 with bluish steatite and amphibole, presents, like almost all 

 the co-ordinate formations of diorite and serpentine (in 

 Silesia, at Fichtelgebirge, in the valley of Baigorry, in the 

 Pyrenees, in the island of Cyprus, and in the Copper Moun- 

 tains of circumpolar America),* traces of copper. AVhere 

 the diorite, partly globular, approaches the green slate of 

 Malpasso, real beds of green slate are found inclosed in 

 diorite. The fine saussurite which we saw in the Upper 

 Orinoco in the hands of the Indians, seems to indicate the 

 existence of a soil of euphotide, superposed on gneiss- 

 granite, or amphibolic slate, in the eastern part oi the Sierra 

 Parime. 



IV. GBANULAR AND MICACEOUS LIMESTONE OF THE 

 MOEROS OF SAN JUAN. The Morros of San Juan rise like 

 ruinous towers in a soil of diorite. They are formed of a 

 cavernous greyish green limestone of crystalline texture, 

 mixed with some spangles of mica, and are destitute of shells. 

 We see in them masses of hardened clay, black, fissile, 

 charged with iron, and covered with a crust, yellow from de- 

 composition, like basalts and amphiboles. A compact lime- 

 stone containing vestiges of shells, adjoins this granular 

 limestone of the Morros of San Juan, which is hollow within. 

 Probably on a further examination of the extraordinary strata 

 between Villa de Cura and Ortiz, of which I had time only 

 to collect some few specimens, many phenomena may be 

 discovered analogous to those which Leopold von Buch has 

 lately described in South Tyrol. M. Boussingault, in a 

 memoir which he has recently addressed to me, calls the 

 rock of the Morros a " problematic calcariferous gneiss." 

 This expression seems to prove that the plates of mica take 

 in some parts a uniform direction, as in the greenish 

 dolomite of Val Toccia. 



Franklin's Journey to the Polar Sea, p. 629. 



2c2 



