Notice of Active and Extinct Volcanos. 28.3 



" We must not however go so far as to suppose, that Tene rifle 

 itself is altogether exempted from those convulsions of nature 

 which are so common in the neighboring islands. 



" Its lofty peak, although it may act as a safety valve, and 

 moderate the violence of the volcanic action by determining it 

 to a point at which it can obtain a vent, proves nevertheless 

 from this very circumstance a dangerous neighbor to the towns 

 that lie underneath it. In the years 1704 and 17u6 lateral erup- 

 tions took place from the Peak the latter of which destroyed the 

 port Garachico, the finest and most frequented harbor in the isl- 

 and. In 1798 too, the mountain Chahorra threw out lavas and 

 scoriae for the space of more than three months, and the violence 

 of the eruption may be judged of by the fact mentioned by 

 Humbolt on the authority of an eye witness ; namely that con- 

 siderable fragments of stones were thrown to such an height, 

 that from twelve to fifteen seconds were reckoned during their 

 descent. This curious observation proves, that rocks were pro- 

 jected from this crater to a height of three thousand feet and up- 

 wards. 



" Before I conclude the subject we are upon, I may remark, 

 how strikingly the difference between the volcanic products of 

 Teneriffe illustrates the manner, in which the effects of heat are 

 modified in such cases by the influence of pressure. 



" At the bottom of the mountain are the basaltic lavas or tuffs, 

 which being produced probably under the ocean, and at a very re- 

 mote period, are compact and possess a stony fracture. Through 

 these have been protruded the trachytes of the peak, which, 

 having had the resistance of so large a body of rock to overcome, 

 also possess a considerable degree of compactness. 



" This conical and upheaved mass having become the centre 

 of the volcanic operations subsequently carried on, is surrounded 

 by products of later formation, some of which were ejected 

 from the summit at a time when a free channel of communica- 

 tion existed between it and the interior of the volcano, others 

 from the flanks at a later period, when the aperture had become 

 obstructed by the falling in of its sides, or the accumulation of 

 ejected substances. It is clear, that in either of these cases, the 

 pressure exerted upon the substance whilst in a melted state 

 was less considerable than that which prevailed during the for- 

 mation of the submarine lavas, or even of the trachyte, and 

 hence it is found to possess more of a vitreous aspect, and is 

 more completely penetrated with cells. The pumice never 

 covers any of the currents of lava, a proof of its greater an- 

 tiquity. See Von Buch. In Leonh. Min. Tasch. 4 part. 1823. n 



Palma presents a very instructive scene. Like the other 



