211 



is called the Caldera, is extremely small ; and 

 this diminutiveness had already struck Mr. de 

 Borda, and other travellers, who took little in- 

 terest in geological investigations. 



As to the nature of the rocks which compose 

 the soil of TenerifFe, we must first distinguish 

 between productions of the present volcano, and 

 the range of basaltic mountains, which surround 

 the Peak, and which do not rise more than five 

 or six hundred toises above the level of the 

 Ocean. Here^as well as in Italy, Mexico, and 

 the Cordilleras of Quito, the rocks of t rap p- for- 

 mat ion * are at a distance from the currents of 

 recent lavas ; every thing shows, that these 

 two classes of substances, though they owe their 

 origin to similar phenomena, date from very 

 different periods. It is important to geology, 

 not to confound the currents of modern lavas, 

 the heaps of basalt, greenstone, and phonolite, 

 which are dispersed over the primitive and 

 secondary formations, with those porphyroid 

 masses with basis of compact feldspar*^, which 



* The trapp-formation includes the basalts, greenstone 

 (grunstein), the trappean porphyries, the phonolites or por- 

 phyrschiefer, &c. 



t These petrosiliceous masses contain vitreous and often 

 calcined crystals of feldspar, of hornblende, of pyroxene, a 

 little of olivine, but scarcely any quartz. To this very am- 

 biguous formation belong the trappean porphyries of Chim- 

 borazo and of Riobamba in America, of the Euganean moun- 

 tains in Italy, and of the Siebengebirge in Germany ; as well 



p 2 



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