Mr. Alison on the Geology of Teneriffe. 437 



is of the same colour, but has a porphyritic character, as 

 it contains crystals of augite and felspar: — these two varie- 

 ties are in the greatest abundance in the Cafiadas del Pico, 

 which in many places are covered to the depth of seventy 

 or eighty feet. The 3rd variety is of a pale olive colour; 

 its surface is rough, but the pores are minute. The 4th is a 

 gray pumice with small veins of carbonate of lime traversing 

 it. The 5th variety hardly ought to be called pumice, as it is 

 a vitreous obsidian which has had the outside changed by the 

 great strength of volcanic heat into a dull rough sort of 

 pumice, with the fibres hardly discernible. 



The 3rd, 4th, and 5th varieties of pumice, I have only 

 seen in the neighbourhood of the Peak; but no doubt they 

 are to be found in other places, as pumice entirely covers most 

 of the plateaux or table-lands of the island (which are only 

 extinct craters of volcanoes, several square miles in extent); 

 and when the mountain torrents break through the lavas, and 

 form ravines of an enormous depth, beds of pumice are fre- 

 quently observed of great thickness. The finest specimens 

 of pumice are to be seen upon a mountain behind the town 

 of Guimar, and I understand they are equal, if not superior, 

 to those of the Lipari islands. 



It appears that the white cinders are thrown out last from 

 a volcano, and announce the end of an eruption. When the 

 heavy compact lavas had ceased flowing, the black rapilli 

 were ejected; after which, the volcanic fires still diminishing 

 in strength, the white rapilli were then thrown out, but to a 

 smaller distance. 



This last-named production surrounds the Peak, and co- 

 vers vast plains some miles from it : but from other volcanoes 

 it seldom extends so far as the sea, nor to a great distance 

 from a crater. The black rapilli, on the contrary, are found at 

 a^considerable distance from any crater, and frequently in thick 

 strata near the sea. 



There are several sorts of tufa, and they generally contain 

 a large proportion of carbonate of lime. One sort is generally 

 found in rather thin beds under numerous strata of lava; it is 

 a brick- red colour, and may be called a puzzalana, as it con- 

 sists of a carbonate of lime with silex and a large portion of 

 iron, and to have flowed in a perfectly fluid state, and turned 

 into a brittle red mass by a stream of hot lava afterwards 

 flowing over it. I have repeatedly observed, that when the 

 stratum of lava over it was thicker in one place than in an- 

 other, the tufa was of the deepest colour, where it had 

 been exposed to the greatest and longest continued heat. The 

 second variety resembles trass or tarras, and is found in si- 

 milar 



