250 REACTION OF THE INTERIOR OF THE EARTH 



the sea, between La Paz and Potosi. The enormous 

 height to which fossils found by Abich in Daghestan, 

 and by myself in the Cordilleras of Peru (between 

 Gruambos and Montan), show the chalk formation to 

 have been lifted, forcibly remind us that non-volcanic 

 sedimentary strata, full of organic remains, not to be 

 confounded with volcanic beds of tufa, show themselves 

 in places where, for a great distance around, mela- 

 phyres, trachytes, dolerites, and other pyroxenic rocks, to 

 which the upheaving impelling forces are attributed, re- 

 main concealed deep below. What vast tracts of the 

 Cordilleras and of the country to the eastward may be 

 explored without any visible trace of any granitic rock ! 

 As I have already, more than once, remarked, the 

 frequency of eruptions in a volcano appears to be de- 

 pendent on varied and very complicated causes; no 

 general law can be established respecting the ratio of 

 absolute height to frequency and intensity of igneous 

 action. If in one small group, the comparison of Strom- 

 boli, Vesuvius, and Etna, might mislead us to suppose 

 that the number of eruptions is inversely proportional to 

 the height of the volcano, we soon find other facts which 

 are in direct contradiction to this supposition. Sartorius 

 von Waltershausen, who has done so much for the 

 knowledge of Etna, remarks that, taking the average of 

 recent centuries, an eruption may be expected about 

 every six years, whereas in Iceland (where, properly 

 speaking, no part of the island can be said to be secure 

 from destruction by subterranean igneous action), Hecla, 

 which is 5755 feet lower than Etna, has, so far back as 

 our knowledge extends, sent forth eruptions only every 

 seventy or eighty years. ( 373 ) The Quito group of 



