436 Mr. Alison on the Geology of Teneriffe. 



and presents a perpendicular wall of aggregates 150 feet high ; 

 this wall is crossed at right angles by dykes of trap, which at 

 a distance give it the appearance of the pipes of an organ. 



Near this spot are large scattered masses of amorphous 

 amygdaloidal lava, filled with crystals of augite, hornblende, 

 idocrase, leucite, felspar and cubicite. 



On the west side of the island, in the valley of Vilna, there 

 are high dykes of trap : the space between these dykes was 

 evidently filled up with breccias at one period, but now they 

 are destroyed (probably by torrents of water from some of the 

 neighbouring volcanoes), leaving isolated walls of trap, which 

 the ignorant natives suppose were erected by the Guanches, 

 or some supernatural beings. The basaltic dykes do not (like 

 those in Scotland and other places) follow a particular point 

 of the compass, but they are flexuous and uncertain. 



The composition of lava is not only extremely various from 

 the same volcano, but from different parts of the same stream : 

 near the crater it will be close, compact, and free from cry- 

 stals ; but if it be traced to some distance below, it will be 

 found frequently to be vesicular, and to contain numerous 

 crystals. 



The lavas are generally in broad streams, from three to 

 twenty feet thick ; but large amorphous blocks are frequently 

 met with, containing numerous crystals of augite, felspar and 

 hornblende, and occasionally olivine. 



Upon some of the small volcanoes called in the country 

 Montaiietas, are found hollow hemispherical masses, which 

 I name, from their appearance, volcanic bombs; they are ge- 

 nerally twelve inches in diameter : the exterior is a compact 

 reddish-coloured lava, but the interior is much less compact, 

 as if following the law of fluid bodies when turning round a 

 centre. They are covered generally on the outside by a white 

 rind, probably caused by the action of sulphureous vapours. 

 Some of the bombs are composed of obsidian to a thickness of 

 three or four inches from the outside, but nearer the interior 

 they present a fibrous appearance showing its passage into 

 pumice. The montaiietas are covered with irregular de- 

 tached pieces of scoriae and volcanic slag ; they are all much 

 tumefied and expanded by sulphureous vapours, and almost 

 as light as pumice. These slags frequently exhibit all the 

 colours of the rainbow, but their general appearance is a dark 

 blue, a black, or a brown, and some so exactly resemble blue 

 iron slag that it is difficult to distinguish them from it. 



There are several varieties of pumice in different parts or 

 the island. — The 1st, and most common, is of a grayish or a 

 grayish- white colour, occasionally tinged with red. The 2nd 



is 



