52 VULCANISM ITS NATURE AND FUNCTION. 



object of the present Sketch is to describe the operations of 

 this internal heat, to explain what is known of its origin, 

 and to define its apparent function in the economy of na- 

 ture. To render a subject intelligible it is necessary to be 

 methodical ; and we shall therefore, without any assumption 

 of technical exactitude, arrange all the thermal phenomena 

 of our globe under the three great heads of Volcanoes, 

 Earthquakes, and Crust-Motions. 



Every one is less or more acquainted with the aspect and 

 nature of a Volcano or burning-mountain. Whether he has 

 seen one or not, his readings and hearings lead him to asso- 

 ciate with his idea a mountain of a conical form, having a 

 crater or orifice of eruption at top, and from which at inter- 

 vals are emitted clouds of vapour and flame, showers of 

 dust and ashes, and streams of lava or molten rock-matter. 

 The outline may not be strictly conical, yet such is the 

 form which it usually assumes ; there may be more craters 

 or orifices than one, some being central or near the top, and 

 others lateral or placed along the sides ; and the sub- 

 stances discharged may be very heterogeneous now highly 

 heated steam and sulphurous vapours, now dust and cindery 

 matters called sconce, now fragments of rock (lapilli and 

 volcanic bombs), and anon wellings-out of lava, sometimes 

 extremely fluid and at others slaggy and cindery. Notwith- 

 standing such local and periodic differences, there is, on the 

 whole, a great similarity in aspect and operation between 

 all volcanoes, which leads to the belief that they belong to 

 one brotherhood, and to the same system of causation. It 

 is usual to speak of lava-cones, tufa-cones, cinder-cones, and 

 mixed cones, according as a mountain is chiefly composed of 

 one or other of these substances, or of a mixture of all of 

 them; but, as might be anticipated, the mixed cones are by 

 far the most prevalent,, and the distinction is mainly valu- 



