chap. y. Breached Craters. 129 



were erupted whilst standing in the sea, we can see 

 why the rule does not apply to them. At Ascension, it 

 was shown that the mouths of the craters, which are 

 there all of terrestrial origin, have been affected by the 

 trade wind ; and this same power might here, also, aid 

 in making the windward and exposed sides of some of 

 the craters originally the lowest. 



Mineralogical composition of the rocks. — In the 

 northern islands, the basaltic lavas seem generally to con- 

 tain more albite than they do in the southern half of the 

 Archipelago ; but almost all the streams contain some. 

 The albite is not unfrequently associated with olivine. 

 I did not observe in any specimen distinguishable 

 crystals of hornblende or augite ; I except the fused 

 grains in the ejected fragments, and in the pinnacle of 

 the little crater, above described. I did not meet with 

 a single specimen of true trachyte ; though some of the 

 paler lavas, when abounding with large crystals of the 

 harsh and glassy albite, resemble in some degree this 

 rock ; but in every case the basis fuses into a black 

 enamel. Beds of ashes and far-ejected scoriae, as pre- 

 viously stated, are almost absent ; nor did I see a frag- 

 ment of obsidian or of pumice. Von Buch 1 believes 

 that the absence of pumice on Mount Etna is conse- 

 quent on the feldspar being of the Labrador variety ; 

 if the presence of pumice depends on the constitution 

 of the feldspar, it is remarkable, that it should be ab- 

 sent in this archipelago, and abundant in the Cordillera 

 of South America, in both of which regions the feld- 

 spar is of the albitic variety. Owing to the absence of 

 ashes, and the general indecomposable character of the 

 lava in this Archipelago, the islands are slowly clothed 

 with a poor vegetation, and the scenery has a desolate 

 and frightful aspect. 



1 ' Description des Isles Canaries,' p. 32S. 



