30 BASALT ALTERNATING WITH MARL. 



The rocks of this small island were of basalt and 

 marl, destitute of trees or shrubs, in most places 

 without a trace of soil, and but scantily crusted with 

 lichens. 



The basalts are not columnar, but arranged in 

 strata from 10 to 16 inches thick, and incline to the 

 north-west 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 di- 

 ameter, which are coated with calcedony, and en- 

 close fragments of compact basalt. The marl, which 

 alternates more than a hundred times with the trap, 

 is of a yellowish colour, extremely friable, A^ery 

 tenacious internally, and often divided into regular 

 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 Schneiben- 

 berger Huegel of Saxony. These rocks were cov- 

 ered 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 black and basaltic, the other white and quartzy. 

 Exposed to the sun's rays the thermometer rose in 

 the former to 124-2°, and in the latter to 104° ; while 

 in the shade the temperature of the air was 81 -5°, 

 being 14° higher than the sea air. The quartzy sand 

 contains fragments of felspar. Pieces of granite 

 have been observed at Teneriffe ; and the island of 

 Gomera, according to M. Broussonet, contains a nu- 

 cleus of mica-slate. From these facts Humboldt 

 infers that in the Canaries, as in the Andes of Quito, 

 in Auvergne, Greece, and most parts of the globe, 

 the subterranean fires have made their way through 

 primitive rocks. 



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



