708 DYNAMICAL GEOLOGY. * 



The evidence that heat has been the promoting cause is as fol- 

 lows : — 



1. The effects are analogous to those which heat is known to produce. — 

 Water, though a weak chemical agent when cold, if heated, has in- 

 creased solvent and decomposing powers and increased efficiency in 

 promoting chemical changes. When heated under pressure above 

 the boiling-point of water (212° F.), it has the qualities, or is in the 

 condition of, superheated steam, and is then an exceedingly power- 

 ful agent as a destroyer of cohesion and solvent, and a promoter 

 of decompositions preparatory to recompositions, as Daubree and 

 others have shown. The moisture disseminated through rocks and 

 distributed among them would be for the most part, if not every- 

 where, in this superheated condition. 



When feldspar, or a related mineral, is acted upon by these 

 means, the waters take its alkalies and silica and become a siliceous 

 solution, fitted to promote solidification or to make new crystalliza- 

 tions ; and when moisture is diffused through a rock containing 

 feldspathic ingredients, this siliceous solution is alike diffused, and 

 a simple cooling may cause it to concrete and solidify the mass. 

 The G-eysers afford an example of siliceous waters formed in this 

 manner. These siliceous solutions, or, more properly, solutions of a 

 silicate of potash or soda, are in a state to promote combinations, 

 and, wherever the conditions are favorable, may aid in the form- 

 ation anew of feldspar and other silicates. 



Crystallizations of epidote, tourmaline, pyroxene, and other species have 

 been formed in the sandstones adjoining trap dikes, through the heat which the 

 trap had when ejected. Near Rocky Hill in New Jersey, also on the Delaware 

 at Lambertsville, and opposite at New Hope, there occur, under these circum- 

 stances, short prisms of black tourmaline half an inch in diameter, along with 

 epidote. The rock has been distinctly baked by the heat in some cases to a 

 distance of a quarter of a mile, and consolidated to a much greater distance. 



A trap dike intersecting the clayey layers, sandstones and coal beds of the 

 island of Nobby, New South Wales, has baked the clayey layers to a flint-like 

 rock to a distance of two hundred yards from the dike, the whole length of the 

 island : the baking effect must have continued much farther, — though the direct 

 evidence is cut off by the river. 



DaubrSe, besides decomposing various silicates by means of superheated 

 steam, has made, in this way, quartz crystals, feldspar, pyroxene, and mica, the 

 crystallization taking place below the point of fusion. 



Through the diffusion of superheated steam at a high tempera- 

 ture, the rocks may have derived increased flexibility, so that a mate- 

 rial otherwise unyielding, as limestone, was flexed under the slowly- 

 acting pressure, without breaking. The effect may have been even 

 greater in some cases, and have produced plasticity, or semi-fusion, 



