TRUE VOLCANOES. 2.55 



of the Himalaya, which we owe to the meritorious labours 

 of B. H. Hodgson, Jacquemont, Joseph Dal ton Hooker, 

 Thomson, and Henry Strachey, the primary rocks, as they 

 were formerly called, granite, gneiss and mica-slate, appear to 

 be visible here also, although there are no trachy tic formations. 

 In Bolivia, Pentland has found fossil shells in the Silurian 

 schists on the Nevado de Antacaua, 17,482 feet above the 

 sea, between La Paz and Potosi. The enormous height to 

 which from the testimony of the fossils collected by Abich 

 from Daghestan, and by myself from the Peruvian Cordil- 

 leras (between Guambos and Montan), the chalk formation 

 is elevated, reminds us very vividly that non-volcanic sedi- 

 mentary strata, full of organic remains, and not to be con- 

 founded with volcanic tufaceous strata, show themselves in 

 places where for a long distance around, melaphyres,trachytes, 

 dolerites, and other pyroxenic rocks, which we regard as the 

 seat of the upheaving, urging forces, remain concealed in the 

 depths. In what immeasurable tracts of the Cordilleras and 

 the districts bordering them upon the east, is no trace of 

 any granitic formation visible ! 



The frequency of the eruptions of a volcano, appearing 

 to depend, as I h:*ve already repeatedly observed, upon mul- 

 tifarious and very complicated causes, no general law can 

 safely be established with regard to the relation of the abso- 

 lute elevation to the frequency and degree o"f the renewal of 

 combustion. If in a small group the comparison of Strom- 

 boli, Vesuvius, and Etna, may mislead us into the belief 

 that the number of eruptions is in an inverse ratio to the 

 elevation of the volcanoes, other facts stand in direct con- 

 tradiction to this proposition. Sartorius von Waltershausen, 

 who has done such good service to our knowledge of Etna, 

 remarks that on the average furnished by the last few centu- 

 ries, an eruption of this volcano is to be expected every six 

 years, whilst in Iceland, where no part of the island is really 

 secure from destruction by submarine fire, the eruptions of 

 Hecla, which is 5756 feet lower, are only observed every 70 

 or 80 years. 49 The group of volcanoes of Quito presents a 

 still more remarkable contrast. The volcano of Sangay, 

 17,000 feet in height, is far more active than the little conical 

 mountain Stromboli (2958 feet) ; it is of all known volca- 



49 Sartorius von Waltershausen, Skizze von Island, s. 103 and 107 



