434 Mr. Alison on the Geology of Teneriffe, 



The lavas proceeding from the ancient volcanoes in the Cen- 

 tre of the island, and particularly those from the Peak, may be 

 divided into three sorts: 1st, basaltic lava of a blueish-black 

 colour, but with an ochrey crust on the outside, very compacts 

 and with a fracture partly conchoidal. This lava appears to 

 be the most ancient, and is generally found near the sea; and 

 when the stratum is thick, it frequently takes the prismatic 

 form, but when it is rather thin, it is less compact, assumes 

 the appearance of common trap, and is very similar to the 

 greenstone of the Salisbury Craigs near Edinburgh: 



The 2nd sort is of a griinsteinic character, of a dark 

 green appearance : it is found in large blocks in the Canadas 

 del Pico, resting upon beds of pumice, which are in places 

 eighty feet thick. 



The 3rd variety is a trachytic porphyry, which forms the 

 walls of the crater upon the top of the Peak; the colour is a 

 brownish red, but externally it is white, from the action of 

 sulphureous vapours upon the argil of the lava. 



The other lavas, though variously modified, maybe classed 

 under two great divisions: 1st, those of a trachytic character, 

 which are compact from being forced through the primitive 

 ejections. And, 2ndly, lavas which are less compact, and have 

 sometimes a vitreous and sometimes a stony appearance : these 

 are generally covered with the last ejected masses, which are 

 always lapilli or white rapilli. 



Frequently the first stratum of the modern lavas, reckoning 

 from below, is a dull trachytic porphyry, covered by rapilli or 

 an earthy conglomerate which alternate several times. The 

 next is generally a cellular augitic lava, containing felspar, 

 more or less decomposed, alternating the same way as the 

 others : and last of all, at the surface is a sort of basaltic trap 

 (similar to what is called whinstone in Scotland) of a deep 

 black colour, but where it is exposed to the atmosphere it is 

 covered with a yellowish- coloured rind. Most of the com- 

 pact lavas strongly affect the magnetic needle, from the quan- 

 tity of titaniferous iron they contain. 



Upon the sea-coast, about five miles to the west of Orotava, 

 are basalts of a regular hexagonal figure ; and near the same 

 spot, at an elevation of about 1 20 feet above the sea, there is a 

 thick bed of clayey volcanic mud, containing quantities of ma- 

 rine shells. It is difficult to account for this phenomenon, other- 

 wise than by supposing that they were drawn into the crater of 

 the volcano through some crevices between it and the sea, 

 and afterwards thrown out with the mud. It is well known, 

 that many of the existing volcanoes have a communication 



"with 



