HEAT. 265 



plate glass, at the Thames Glass Works, the 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 decreases in density 9 to 11 

 per cent ; syenyte, 8 to 9 ; dioryte, 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). 



III. IGNEOUS ACTION AND ITS RESULTS. 



Igneous action has its origin almost exclusively within the earth's heated 

 interior. A few phenomena only are due to exterior agencies. Its chief 

 direct results include : (1) the melting of rocks ; (2) the eruption of melted 

 or plastic rock from some subterranean source into or through fissures or 

 spaces opened in the earth's crust, — thus making eruptive rocks; (3) the 

 repeated eruption of melted rock, through long periods, from local vents, — 

 thereby making volcanoes ; (4) the imbibing by the melted rock, while on 

 its way up, of vapors generated from ingredients encountered in the adjoin- 

 ing rocks, and especially of water-vapor, derived from the moisture of these 

 rocks and from subterranean streams, — producing, in the melted rock, aug- 

 mented mechanical and chemical powers ; (5) the communication of heat 

 and vapors to the adjoining rocks, — producing in these outside rocks chemi- 

 cal and physical changes. Earthquakes, solfataras, fumaroles, hot springs, 

 geysers, and also mineral depositions and emanations in connection with the 

 hot springs and fumaroles, are among the attendant results. 



In the following pages the results of exterior agencies are first presented ; 

 and then those of interior origin, under the heads of Volcanoes, Non-volcanic 

 Igneous Eruptions, and Geysers. 



Action of Exterior Agencies. 



Lightning, an electric discharge or a combination of them, occasionally 

 leaves evidence of its intense heat on rocks and sand-heaps, by the 

 fusion of the constituent minerals into a tube around its pathway, or in 

 patches of glassy beads. The tubes, called fwlgurites, have been observed K 

 in many places in the sands of dunes, descending to a depth of one to three 

 feet ; and one of ten feet is reported. They are one half to two or more 

 inches across, often contorted, taper, and sometimes branch, downward. 

 Tubes two feet long, found near Pensacola, Fla., consisted within of a 

 bright clear glass almost free from grains of quartz (Diller, 1884). A 

 fulgurite from the sand near Waterville, Me., has been described by W. S. 

 Bayley (1872). The fulgurites in rocks occur especially about the summits 

 of mountains. They have been observed in Mexico in the trachytic summit 

 of Toluca (Humboldt) ; in Little Ararat, Caucasus, in augite-andesyte 

 (Abich); on the top of Mont Blanc and at a dozen other points in the Alps ; 

 at many places in the Pyrenees ; also in Oregon and Colorado. 



