222 



METAMOKPHIC ROCKS. 



feldspar, mica, augite, etc., are formed. In fact, as Guthrie has shown 

 (Geological Magazine, vol. vi, p. 244, 1889), there are all gradations 

 between solution and true igneous fusion through various grades of 

 hydrothermal fusion. Such a pasty or aqueo-fused mass slowly cooled 

 would form a crystalline rock containing crystals of quartz, feldspar, 

 mica, etc. ; in other words, would be metamorphic. The quantity of 

 water necessary for these effects is shown by experiment to be very 

 small — only five to ten per cent. In other words, the included ivater 

 of sediments is amply sufficient. 



Alkali. — Alkaline carbonates, or alkaline silicates, so common in 

 natural waters, greatly promote the process, causing the aqueo-igneous 

 pastiness or aqueo-igneous fusion to take place at a much lower tem- 

 perature. 



Pressure. — Pressure is a necessary condition of the existence of 

 high temperature in the presence of water, and is thus an indirect 

 agent of metamorphism, but it is also a direct agent, since it increases 

 chemical action of many kinds, and therefore solubility. 



It is evident, therefore, that while metamorphism by dry heat would 

 require a temperature of 2,000° to 3,000° Fahr., in the presence of water 

 the same result is produced at 572° to 752° Fahr. (300° or 400° C.) ; or 

 in the presence of alkali, even in small amount, probably at 300° or 

 400° Fahr. 



Application. — All these agents are found associated in deeply-buried 

 sediments. Series of outcropping strata are often found 20,000 or even 

 40,000 feet thick. The lower strata of such a series by the regular 

 increase of interior heat alone, must have been, before uptilting, at a 

 temperature of between 700° and 800° Fahr., a temperature sufficient, 

 with their included water, to produce complete aqueo-igneous pastiness, 

 and therefore, by cooling and crystallization, complete metamorphism. 

 Suppose, then, «, sb, Fig. 192, represent the contour of land and 

 sea-bottom at the beginning of any period, and the dotted lines the 



isogeotherm of 

 400° and 800°. 

 If, now, sedi- 

 ments 40,000 to 

 50,000 feet thick 

 be deposited so 

 that the sea-bot- 

 tom is raised to 

 s'b\ then the iso- 

 therm of 800° 



will rise to the position of the broken lines and invade the lower por- 

 tions of the sediments with their included water. Such sediments 

 would be completely changed in their lower portions, and to a less ex- 



.... ■:■■■■-■■- 



••■ ■■'■. .■■<•■ ■■■■■•... 



Fig. 192. — sb, original sea- bottom; s'b', sea-bottom after sediments, 

 have accumulated: . . . ., isogeotherms of 800" and 400° : — . — 

 same after accumulation of sediments. 



