NATURAL HISTORY. 



603 



of a few leagues I counted no less 

 than seven cones of extinct volca- 

 noes, and the country is covered 

 with scoria, exhibiting no appear- 

 ance of culture, and iiardly any of 

 vegetation ; it is more broken into 

 ravines and more intersected by 

 lava torrents than on any of the 

 other sides of the island. Numer- 

 ous peaked and conical mountains 

 rise upon the slope of the chain, 

 and the whole country is covered 

 by scoria, and is one continued 

 stream of lava. The Montana 

 Roxa itself is a singular example 

 of the dislocation of strata so com- 

 monly found in countries of volca- 

 nic formation ; it is evidently' a slip 

 or fall of semi-columnar lava, and 

 slopes into the sea at an highly in- 

 clined angle. 



The ordinarj^ strata of the island 

 are as follows, reckoning from be- 

 low upwards: 1st. the porphvriiic 

 lava covered by scoria and some- 

 times by pumice. This lava is 

 composed of hornblende and feld- 

 spar, and contains no other sub- 

 stance. The next stratum gradu- 

 ates into what the Spaniards call 

 Roccaverde or greenstone, and is 

 composed of feld»par and horn- 

 blende; upon this is generally a 

 thick stratum of pumice, and last 

 of all towards the surface is the ba- 

 saltic lava covered also by tufa and 

 ash. This lava decomposes the 

 soonest. It also contains the great- 

 est variety of extraneous substances, 

 and is sometimes divided l)y a layer 

 of large crystals of olivine some 

 inches long, and towards the north- 

 east is often intersected by stata 

 of porjjhyritic slate. These lavas 

 are more earthy and cellular than 

 those which I have had an oppor- 

 tunity of observing elsewhere, yet 

 thev contain fewer extraneous sub- 



stances than those of TEtna and 

 Vesuvius ; they are in some places 

 exposed to view in the vallevs si- 

 milar to those of the Corral iii the 

 island of Madeira. The valley of 

 Las Gnanchas on the north-west 

 side of the Peak, contains, accord- 

 ing to M. Escolar, above 100 strata 

 of lava, the one reposing upon the 

 other, at times alternating with 

 l)umice and tufa. The depth of 

 these strata varies. M. Escolar has 

 seen one of basaltic lava between 

 100 and 150 feet in depth in one 

 solid mass, cellular at the sur- 

 face, but gradually becoming more 

 compact towards the bottom. This 

 basaltic lava contains olivine and 

 hornblende, and in the caves on the 

 coast, zeolite. This substance is 

 also found in stalactites and in 

 masses, sometimes in layers spread 

 Ijetwf-en the strata and diffused 

 over the rock. 



Nodules of chalcedony are some- 

 times also found, but these sub- 

 stances occur only in the chain of 

 mountains towards the north-e:ist, 

 from the northern extremity of 

 Santa Cruz to the point of Hidalgo. 

 Tlie lavas of the island are of an 

 endless variety, and the number of 

 streams that have flowed are much 

 beyond all enumeration. The 

 whole surface is either ash, or solid 

 or decomposed lava, which seems 

 again and again to have been per- 

 forated by volcanic eruptions ; the 

 number of small extinct volcanoes 

 is prodigious, they are to be found 

 in all parts of the island, but the 

 stream that has flowed from even 

 the largest of them, such as the 

 lavaof thePeak called ElMal Puis, 

 is trifling in comparison with that 

 immense mass of lava mountains 

 which constitute the central chain 

 of the island, and which stretch out 



