. Elements of geology : a text-book for colleges and for the general reader. Geology. 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 smallâon
. Elements of geology : a text-book for colleges and for the general reader. Geology. 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 smallâonly five to ten per cent. In other words, the included ivater of sediments is amply sufficient. Alkali.âAlkaline 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.âPressure 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.âAll 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, t
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