32 



GEOLOGICAL OBSERVATIONS. 



Basaltic 

 rocka 



Different 



ciiAi'. II. at an angle of 80 degrees, alternating with marl. Some 

 of these strata are compact, and contain large crystals of 

 foliated olivine, often porous, with oblong cavities, from 

 two to eight lines in diameter, which are coated with 

 calcedon y, and enclose fragments of compact basalt. The 

 marl, which alternates more than a hundred times witli 

 the trap, is of a yellowish colour, extremely friable, 

 very tenacious internally, and often divided into irre- 

 gular prisms like those of basalt. It contains much 

 lime, and effervesces strongly with muriatic acid. The 

 travellers had not time to reach the summit of a hill, the 

 base of which was formed of clay, with layers of basalt 

 resting on it, precisely as in the Scheibenberger Huegel 

 of Saxony. These rocks were covered with hyalite, of 

 which they procured several fine specimens, leaving 

 masses eight or ten inches square untouched. 



On the shore there were two kinds of sand, the one 

 l)lack and basaltic, the other white and quartzy. Ex- 

 posed to the sun's rays the thermometer rose in the 

 former to 12-i-2°, aitd in the latter to 10-4° ; while in the 

 shade the temperature of the air was 81'8°, being 13'5° 

 higher than the sea air. The quartzy sand contains 

 fragments of felspar. Pieces of granite have been olj- 

 served at Teneriffe ; and the island of Gomera, according 

 to M. Broussonet, contains a nucleus of mica-slate. 

 From these facts lIuml)oldt infers, that, in the Canaries 

 as in the Andes of Quito, in Anvcrgne, Greece, and most 

 ])arts of the globe, the subterranean fires have made their 

 way through i)rimitivc rocks. 



Having re-embarked, they hoisted sail, and cndea- 

 vnured to get out again by the strait whicli separates 

 Alcgranza from Montana Clara; but, the wind having 

 fallen, tlie currents drove them close upon a rock marked 

 in old ciiarts I)y the name of Infierno, and in modern 

 ones under that of Roca del Oeste, — a basaltic mass 

 which has prol)ably l)een raised by volcanic agency. 

 'Jacking during tlie night between Montana Clara and 

 this islet, tliey were several times in great danger among 

 «hi;lve8 towards which they were drawn by the motion 



Clianfjc of 



wimL 



