METAMORPHISM. 727 



and not metamorphic ; and in Nova Scotia the coal formation, though 15,000 feet thick, 

 is not metamorphic at base. Taking the increase of temperature in the earth's crust at 

 1° F. for sixty feet of descent, 10,000 feet of depth would give 220° F. as the tempera- 

 ture of the limestone before the faulting, and 15,000 feet, 314° F. But 1° F. per sixty- 

 feet of descent is the present rate, and must be far short of that at the close of the 

 Carboniferous age, when the earth's crust was so easily flexed, and metamorphism took 

 place on so grand a scale; and hence the limestone must have been subjected to a heat 

 far above 220° F., if at a depth of 10,000 feet. 



Moisture is essential, because dry rock is a non-conductor of heat 

 (as well shown in the case of a common fire-brick), and also because * 

 of its chemical powers when heated. Rocks usually contain some 

 moisture ; and, when moist, heat goes rapidly through them. 



The pressure may have been that of either superincumbent waters 

 or of overlying rocks. A little thickness of the latter would give all 

 the pressure that was in any case essential. 



The evidence that heat has been a promoting cause is as follows : — ■ 



1. Tlie 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. As stated on pag£ 719, it becomes 

 siliceous ; and, at high temperatures, it is an exceedingly powerful agent 

 as a destroyer of cohesion, a 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 everywhere, in a superheated con- 

 dition. When moisture is diffused through a rock containing feld- 

 spathic ingredients, the siliceous solution is alike diffused, and is in a 

 state to promote combinations, and, wherever the conditions are favor- 

 able, may aid in the formation anew of feldspar and other silicates. 



Crystallizations of epidote, tourmaline, garnet, chlorite, and hematite have been 

 formed in the sandstones adjoining the trap dikes intersecting the Triassico-Jurassic 

 red sandstones of the Atlantic Border of North America, through the heat which the 

 trap had when ejected. These are examples of local metamorphism; but still they are 

 good illustrations of the changes in regional metamorphism. 



A trap dike intersecting the clavev 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. 



Daubree, 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 temperature, 

 the rocks may have been rendered even plastic ; and, in this condition, 

 limestone might have been pressed into fissures in adjoining rocks, so . 

 as to make a kind of injected vein. The preservation in nearly all 



