550 MADEIRA. 
Pico Ruivo is the highest point on the island, being 6237 feet above 
half tide, according to the barometrical observations of the Expedi- 
tion.* Over the western half of the island, the central ridge (Paul de 
Sierra) is less broken than to the east; it has been ascertained to be 
5194 feet high. There are extensive forests of heath and broom scat- 
tered over the heights; for the heath grows to the stature of trees, or 
a height of thirty feet, and the broom attains nearly half this size. The 
eastern extremity of the island is a narrow ridge of rock, partaking of 
the general character of the island, though much less elevated. 
The valleys usually enclose a strip of cultivated land between their 
high, precipitous sides, watered by a streamlet, which becomes a 
mountain torrent in the wet season, though it may be nearly dry in 
summer. ‘The latter was the condition about the middle of September 
of 1838, when we spent five days on the island. 
Rocks.—Madeira consists throughout of volcanic rocks, excepting 
small deposits of tertiary and recent limestone. 
The igneous rocks are mostly a compact grayish basaltict lava, a 
scoriaceous rock of similar composition, and different varieties of tufa 
and coarse conglomerate. ‘The basalt is generally tough, with few 
cellules and a bluish-gray colour: but a brownish-red shade is also 
common, and characterizes whole cliffs. The texture is rarely at all 
crystalline, and the rock, therefore, breaks with a smooth surface, and 
usually a conchoidal fracture. It contains, however, grains of olivine, 
and occasionally distinct crystals of feldspar or augite. ‘The olivine 
is sometimes very profusely disseminated. The augite occurs in large 
black crystals, which are quite brilliant on the surface of perfect 
cleavage. 
From the compact basalt there are insensible gradations to a perfect 
scoria of a browntsh-red colour. 
In some specimens obtained from the bed of the Socorridos, the 
rock was a vesicular trachytic rock, containing thickly disseminated 
tabular crystals of feldspar. Another vesicular variety contained the 
three minerals, feldspar, augite and olivine. ‘The olivine had often a 
submetallic lustre when broken, arising from incipient decomposition. 
Obsidian and pumice were met with only in beds of tufa. Some of 
the vesicular varieties contained stilbite in its cavities. 
* The altitude, as determined by Captain Ross, in 1839, is 6097-08—6102:90 feet. 
(Voyage to the Southern Seas, 1839—43. London, 1847 ; vol. i. 6.) 
+ We still use the word basaltic in a general way, in preference to more specific terms, 
for a rock containing augite and feldspar, and often olivine. 
