14 Teneriffe. [Jan., 
remained there an hour to breakfast and to enable me to make 
the following observations :— 
Tn. 
The Barometer stood at : : : . w0sobe 
Thermometer in the shade . : . ae 
Wet-bulb Thermometer < ; , 4 
Black ditto : f , 3 . 140 
Boiling point of water . ‘ : 2 LOG 
I am not certain that this is perfectly correct, as it varied from 
196°°4 to 196°°7, the bulb of the thermometer was not plunged 
into the water, but in the steam about an inch above the surface of 
the water. 
We began to mount the Peak on foot, by a very steep 
ascent over a surface covered with yellowish coloured pumice, 
between two embankments or currents of trachytic lava, which 
had separated in cooling from the general mass called Mal Pais, 
situated at Alta Vesta de Arriba 10,621 feet above the sea. It is 
not quite correct to call them currents; although in continuous 
lines, the lava was not in connected masses, but in large detached 
blocks of various sizes and forms, that had apparently undergone 
various degrees of rapid and slow cooling; the most common 
description was a trachy-dolerite, with more or less felspathic 
minerals ; some were obsidian, or volcanic glass, of a jet-black colour, 
having internally a shining vitreous lustre, breaking with a con- 
choidal fracture, and translucent at the edges. It is worthy of 
remark, that when I broke some specimens from the lowest part 
of a mass of obsidian they contained crystals of felspar, but in the 
upper part of the same mass, very frequently I could not discover 
a single crystal, as if they had fallen down to the bottom, when 
the mass was liquid, by their own specific gravity. On breaking 
some of them, they exhibited cavities contaiing an incipient 
crystallization forming concentric lamine of a lighter colour than 
the rest. In some places I picked up pieces which were slightly 
convoluted, containing small crystals of pyroxene and glassy felspar. 
Another singular circumstance is, that the pumice taken from 
the Cafiadas of the Peak is heavier and contaims more silica and 
alumina, and less potash and soda, than that on the sides and 
top of the Peak; the latter contained about the same quantity of 
silica and alumina as the trachy-dolerite. 
The following is the analysis made for me by the late Dr. 
Andrew Ure :— 
Silica. Alumina, Potash. Soda. 
Trachy-dolerite, from the Peak. | 57 16 95 1-14 6:40 
PAIICOMCIELO Ve.) ie oueat, va fi acertake 59°87 15°89 | 10:20 3:15 
Ditto Catadas. . . . . «| 62°40 | 14:97 | (6-25 1:96 
