336 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



by the sun upon the fluid interior;, and the consequent pres- 

 sure excercised by the latter on the earth's crust. It moreover 

 appears from investigations made by Perrey subsequent to those 

 above mentioned, that the moon also exercises an influence.* 



Quenstedtf thus refers to the latter investigations: " A. Perrey 

 "has found from 1000 observations during the first half of the 

 " present century that earthquakes are much more frequent in the 

 "conjunction and opposition of the moon than at other times; 

 tl more frequent, when the moon is near the earth than when it is 

 " distant, more frequent in the hour of its passage through the 

 ' meridian than at any other. From this it would appear, that 

 "the moon is not without influence ; that it occasions tides in the 

 " central lava in the same manner as in the ocean, which tides 

 " press against the earth's crust and seek an outlet." The latter 

 part of this quotation seems to contain the explanation least liable 

 to objection, of the rise of the lava in volcanoes. Not only may 

 plutonic and volcanic earthquakes be attributed to this cause, but 

 volcanic eruptions also, and with equal justice. If we adopt this 

 explanation it is easier to comprehend why the lava should press 

 forth at one in preference to another volcano, or burst open the 

 obstructed canals of extinct volcanoes rather than seek an outlet 

 through the vents already existing in the earth's crust. 



Having thus discussed the various explanations of the cause of 

 the rise of the lava in volcanic canals, the phenomena which attend 

 volcanic eruptions may next be adverted to, with the view of 

 ascertaining the origin of certain volcanic products. The lava 

 gradually ascending from the depths of the earth, comes in the 

 upper part of the canal and in the crater into contact and conflict 

 with the water, which has found its way down from the surface, 

 and which may have collected in subterranean reservoirs, or merely 

 saturated the side walls of the vent. The water is by the heat 

 of the lava resolved into steam, which then forces its way through 

 the fluid to the surface, when the bubbles containing it explode. 

 A similar phenomenon may be observed on a small scale at blast 

 furnaces, when the slag runs out of the breast and over a place 

 upon which water had been previously thrown. The slag boils up 

 until it cools, and becomes too stiff to allow of the passage of the 

 steam. This production and escape of steam in the craters of 

 volcanoes takes place with a violence and intensity of which few 



* Bronns Jahrbuch, 1855, 12 

 t Epochen der Natur, p. 812. 



