80 C A N A R Y I S L A N D S . 



On Graciosa, there are beds of mar], containing large quanti- 

 ties of chalk which strongly effervesces with nitric acid, 

 " and even on points where it is found in contact with the 

 basalt." * Near the foot of the mountain of Tagayga 

 between Realejo de Abaxo and San Juan de la Rambla, 

 I am told that there are found a species of marble, and 

 some vegetables, either incrusted with calcareous sand, or 

 are petrified. Teneriffe, also, exhibits among its alluvial 

 deposits, an abundance of clayey calcareous tufa, which 

 alternates with volcanic brecci.'E, and which, according to 

 Mr. Viera,t contains, near San Juan de la Rambla, and 

 Candelaria, plants, impressions of fishes, buccinites, and 

 other fossil marine productions. Just on the borders of 

 El Llano de Gaspas, I observed a great number of the 

 impressions of ferns on some horizontal beds of basaltes, 

 and in a stratum of brecciae in a ravine between Santa 

 Ursula and Port Orotava, I observed several recent terres- 

 trial shells, (Helix?) 



Geology and Mineralogy of Teneriffe. In travelling 

 from the sea-shore to the summit of the mountains, the 

 lowermost stratum that we meet with, is composed of huge 

 masses of yellowish-brown basaltes, generally columnar, 

 and somewhat irregular in their forms, compact and par- 

 tially decomposed. They contain hornblende, olivine, and 

 translucent pyroxenes, the latter being of a perfectly lamel- 

 lar fracture, of a tender olive-green tint, and often crys- 

 talized in six-sided prisms. The first of these substances 

 is extremely rare on this island, and very seldom occurs in 

 the modern lavas. Directly above these ancient basaltes, 

 there are strata of breccise resembling volcanic tufa, which 

 are found on both sides of the island, situated about 300 

 or 400 feet above the ocean. They contain fragments of 

 the same basaltes that they cover, and often recent terres- 

 trial shells ; and it is asserted by Mr. Viera, that the 

 remains of marine petrifactions are found in them. Next 

 in succession abov.e, is a stratum of ferruginous earth, over- 

 laid with a dark argillaceous soil, containing crystals of 

 pyroxene, and fragments of compact lava. Next comes a 



* Humboldt. f Noticias Ilistoricas de las Canarias, 1. i. p. 35. 



