617 



M. Boussingault, in a very instructive memoir 

 which he has recently addressed to me, calls 

 the rock of the Morros a " problematic calca- 

 riferous gneiss.** This expression seems to prove 

 that the plates of mica take in some parts an 

 uniform direction, as in the greenish dolomie of 

 Val Toccia. 



V. FELSPATHIC SANDSTONE OP THE OROONOKO. 



The soil of gneiss-granite of the Sierra Parime 

 is covered by fragments, (between the Encara- 

 iii ad a and the strait of Baraguan, and in the 

 Island of Guachaco), in its western part, of an 

 olive-brown sandstone, containing grains of 

 quartz, and fragments of feldspar, joined by 

 a clayey-cement, extremely compact. This 

 cement, where it abounds, has a conchoid 

 fracture, and passes to jasper. It is crossed by 

 small veins of brown iron-ore, which separate 

 into very thin plates, or blades. (Vol. iv, p. 

 573.) The presence of feldspar seems to indi- 

 cate that this small formation of sandstone (the 

 sole secondary formation hitherto known in the 

 Sierra Parime), belongs to red-sandstone or 

 coal *, I have hesitated to join it to the sand- 



* Broken or intact crystals of feldspar are found in the 

 tote liegende, or coal sandstone of Thuringia (Freiesleben 

 geogn. Arbeiten, Vol. iv, p. 82, 85, 96, 194). I observed in 



