722 



DYNAMICAL GEOLOGY. 





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

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

 doleryte, 5 to 7 ; trachyte, 3 to 5 per cent. Cast iron loses in density 

 on heating, and also on solidifying; trials gave a density of 7*214 

 when cold, 6'535 before fusion, and 6*883 when liquid (Hannay). So 

 with lava ; i*i hot crust floats. (See page 810.) 



The following are (he titles of works and memoirs referred to in the preceding pages 

 in addition to those already mentioned: — 



C. Babbage : Observations on the Temple of Serapis, 8vo, 1847, tr^c Geol. Soc.. 

 ii. 73 (having been read before the Society in 1834), anruunces the view of an upward 

 rise in isogeothermal planes as a consequence of the accumulation of sediments, which 

 principle was also independently brought forward by Scrope, in his work on Volcanoes. 



R. Mallet: On Volcanic Energy, Phil. Trans., clxiii., 1873 ; clxv., and Proc. Roy. 

 Soc, xxii. 328, 1872; Phil. Mag., IV. xlviii., 41, 1874, and 1., 122, 201, 1875. Also In- 

 troduction to Palmieri on the Eruption of Vesuvius in 1872, published in 1873. (Amer. 

 J. Sci., III. v., 219, 1873; vii., 145, viii., 140, 1874; x., 256, 1875; a Review by E. W. 

 Hilgard, ibid., vii., 535.) 



Delesse : Bull. Soc. Geol. de France, II. iv., 1380, 1847. 



J. G. Totten, U. S. Engineers: Experiments on the Expansion and Contraction of 

 Building Stones, by variations of temperature. Am. J. Sci., xxii., 136, 1832. 



3. IGNEOUS ACTION AND RESULTS. 



1. Volcanoes. 



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

 lowing heads : (1) General nature of volcanoes, arid their geograph- 

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

 (4) Non-volcanic igneous eruptions ; (5) Heat of lavas, and condi- 

 tions of volcanic action ; (6) Thermal waters, geysers. 



1. General Nature of Volcanoes, and their Geographical 

 Distribution. 



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

 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 



