388 THE CANADIAN NATURALIST. [DeC. 



mena, and to consider the reason of tlieir peculiar geographical 

 distribution. Violent movements of the earth's crust are confined 

 to certain regions of the globe, which are at the same time 

 characterized by volcanic activity ; from which it is reasonably 

 inferred that the phenomena of earthquakes and volcanoes have a 

 common origin. The discharge through openings in the earth's 

 crust of ignited stony matter, generally in a fused condition, and 

 the disengagement of various gases and vapors, accompanied by 

 movements of elevation or subsidence of considerable areas of the 

 earth's surface, sometimes rapid and paroxysmal, and attended 

 with great vibratory movements, are evidences of a yielding crust 

 of solid rock resting upon an igneous and fluid mass below. To 

 the same conditions are also to be ascribed the slow movements 

 of portions of the earth's surface shown in the rise and fall of 

 continents in regions remote from centres of volcanic activity. 

 The unequal tension of the yielding crust and the sudden giving 

 way of the overstrained portions are probably the immediate 

 cause of earthquake phenomena ; the seat of these, according to 

 the deductions of Mallet, is to be found at depths of from seven 

 to thirty miles from the surface. 



A brief description of the phenomena of volcanoes will be 

 necessary as a preliminary to the inquiry which constitutes the 

 object of our lecture. Volcanoes are openings in the earth's crust 

 through which are discharged solid, liquid, and gaseous matter, 

 generally in an intensely heated condition. Sometimes the ejected 

 material is solid, and consists of broken comminuted rock, or the 

 so-called volcanic ashes. Oftener, however, it is discharged in a 

 more or less completely fused condition, constituting lava, which 

 is sometimes fluid and glassy, but more frequently pasty 

 and viscid, so that it flows slowly and with diificulty. The 

 ejected materials, whether liquid, or solid, build up volcanic cones by 

 successive layers — a fact which has been established by modern 

 observers in opposition to the notion come down from antiquity, 

 that volcanic hills are produced by an uprising or tumefaction of 

 previously horizontal layers of rock by the action of a force from 

 beneath. First among the gaseous products of volcanoes is 

 watery vapor ; water appears not only to be involved in all 

 volcanic eruptions, but to be intimately combined with the lavas, to 

 which, as 3crope has shown, it helps to give liquidity. The water 

 at this high temperature is retained in combination under great 

 pressure, but as this pressure is removed passes into the state of 



