Mr. Alison on the Geology of Teneriffe. 435 



with the sea. Humboldt mentions that some of the Andes 

 frequently throw out vast quantities of water, and sometimes 

 mixed with fish. In 1824, a volcano in Lanzerote sent forth 

 a large body of salt water, which did considerable damage to 

 the surrounding lands. 



Columnar lava is found not only near the coast, but like- 

 wise in several ravines at considerable elevations, and even in 

 theCanadas del Pico, which are eight thousand feet above the 

 level of the sea. I have particularly observed that whenever 

 the stratum of lava is thick, it appears to have a constant ten- 

 dency to take the prismatic form ; and little difference appears 

 to exist, whether it cooled quickly or slowly, as the basalt 

 near the sea, proceeding from a crater four or five miles off, 

 and flowing over an inclined plane of five degrees and a half, is 

 equally crystallized [?] as the basalts from a crater only a few 

 hundred yards off. Occasionally the lower part of the stra- 

 tum is perfectly crystallized [?], whilst the upper surface is with- 

 out the slightest sign of crystallization [?], and only presents 

 a close and compact mass. These basalts are internally of a 

 blueish slate-colour or a black, sonorous, compact and hard, 

 and frequently include small crystals of greenish augite, and 

 ■leucite of a vitreous lustre. 



The columns are generally from one to two feet in breadth, 

 sharply defined, and frequently destitute of articulations; very 

 few of them are straight, but are bent in the middle like the 

 ribs of a ship, as if unable while in their soft state to support 

 the mass of incumbent matter. They appear to decompose 

 quickly by the action of the atmosphere ; and from the iron 

 they contain, all the prisms are covered by a yellowish-co- 

 loured coating. 



There is a species of basaltic trap exactly similar in com- 

 position to the columnar basalt ; but I do not recollect hav- 

 ing seen it in thick strata, but generally traversing volcanic 

 breccias in dykes of four or five feet in thickness, nearly per- 

 pendicular to the horizon. These aggregates contain a mass 

 of rounded pieces of lava, strongly calcined, and large cry- 

 stals of hornblende; the basis or cement is generally a tufa or 

 brown mud. 



It is worthy of observation, that these dykes are generally 

 found nearly at the same elevation ; that is, from four to five 

 thousand feet above the level of the sea ; occasionally they are 

 to be met with higher, but seldom lower. 



In the south-east side of the valley of Orotava, at a place 

 called los Organos, is the crater of an extinct volcano, which 

 evidently has fallen in and formed a small but picturesque 

 valley called Agua Mansa : one side of the crater still remains, 



3 K 2 and 



