702 DYNAMICAL GEOLOGY. 



Mallet ; and the results of the three investigators last mentioned 

 nearly agree. In solidification, the glass state is a consequence of 

 rapid cooling, and the stone state, of slow ; and, consequently, glass 

 will become stone, if melted and very slowly cooled. 



In passing from the liquid to the glass state, in the case of plate 

 glass, at the Thames Glass Works, the cubic contraction was 1-59 per 

 cent, — 100 parts, by weight, becoming 98*41 (Mallet). In passing 

 from the glass to the stone state, according to Delesse, granite de- 

 creases in density 9 to 1 1 per cent. ; syenyte, 8 to 9 ; diory te, 6 to 8 ; 

 doleryte and melaphyre, 5 to 7 ; basalt and trachyte, 3 to 5 per cent. 1 

 The whole increase of density for doleryte, in passing from the liquid 

 to the stone state, would be near 8 per cent., which is equivalent to a 

 change of volume from 100 to 92 per cent. 



3. IGNEOUS ACTION AND KESULTS. 

 1. Volcanoes. 



The facts relating to volcanoes are here presented under the follow- 

 ing heads : (1) General nature of volcanoes, and their geographical 

 distribution ; (2) Kinds of volcanic cones ; (3) Volcanic action ; (4) 

 Origin of the forms of volcanic cones ; (5) Subordinate volcanic 

 phenomena; (6) Source of volcanoes. 



1. General Nature of Volcanoes, and their Geographical 

 Distribution. 



1. Volcanoes. — Volcanoes are mountains or hills, of a more or 

 less conical shape, in a state of igneous action, and consequently 

 emitting vapors and, occasionally, melted rock, or lava, with showers 

 of fragments, or cinders, from a central opening, called the crater. They 

 are conduits of fire, opening outward from within or beneath the 

 earth's crust. An extinct volcano is a volcanic mountain that has 

 ceased to be active, — the body, with the fire out. 



The lavas flow out either over the edge, or lip, of the crater, or, 

 more commonly, through fissures in the sides, or about the base of 

 the mountain. The cinders are thrown upward from the vent, or 

 crater, to a great height, as a jet of sparks or fiery masses, and fall 

 around in cooled particles or fragments, which are simply granulated 

 lava : they may build up a conical elevation around the vent, or be 

 carried to a distance by the winds. 



When rain or moisture from any source descends with the cinders, 

 the mass forms tufa, — a kind of volcanic sandstone, being stratified, 



1 Mallet, on Volcanic Energy, Trans. Roy. Soc, 1872; Delesse, Bull. Soc. Geol. de 

 France, II. iv. 1380, 1847. 



