744 A TREATISE ON METAMORPHISM. 



or gneisses, 1.50 per cent or more of water remained. The percentages of 

 combined water after metamorphism are, of course, no measure of the 

 amount of water, free and combined, contained by the rock before and 

 during the metamorphosing process. It is highly probable that the amount 

 of combined water in the later stages of the process, in the case at least of 

 the sedimentary rocks, is lower than in the earlier stages of the process, 

 consequently water is driven off during the process, thus renewing the water 

 films in the subcapillary spaces, and furnishing a medium for solution 

 and redeposition. (See Chapter III, p. 145.) This is inferred from the 

 analyses of pelites in their various stages of alteration from soils and clays 

 to shales, slates, schists, and gneisses. Sixteen analyses given by Clarke 

 and Hillebrand of soils and clays from Pennsylvania, Florida, and Colorado 

 gave an average loss of water at or above 100° C. of 7.15 per cent. Six 

 analyses of clays and soils from Virginia gave an average loss of water 

 above 110° C. of 8.61 per cent. Forty -four analyses of clays and soils 

 from Massachusetts, South Carolina, Alabama, Missouri, Colorado, Nevada, 

 and California gave an average loss of water upon ignition of 8.10 per 

 cent." Twenty slates and shales from Vermont, Colorado, ar I California 

 gave an average of 4.42 per cent of water above 100° C. Fourteen slates 

 and shales from Vermont, New York, Kentucky, Georgia, and Alabama 

 gave an average of 4.34 per cent of water above 110° C. 6 The original 

 mudstones from which the shales and slates were produced may be pre- 

 sumed to have contained at least as much water as do soils and clays. The 

 shales and slates are rocks partly metamorphosed during mass-mechanical 

 action, and they certainly lost about one-half of the combined water during 

 the change. 



It is well known that shales and slates, when profoundly metamor- 

 phosed during mass-mechanical action, produce schists or gneisses. Since 

 most of the schists contain less than 2 per cent of water, this agent was 

 again reduced to one-half or one-third of the amount present when the 

 rocks were in the stage of shales and slates. Therefore the pelites in their 

 original form contain in combination not only a sufficient amount of water 

 to satisfy the requirements as to combined water in the completely meta- 

 morphosed schists, but an excess of water, which may be steadily given off 



a Clarke, F. W., and Hillebrand, W. F., Analyses of rocks and analytical methods: Bull. U. S. 

 Geol. Survey, No. 148, 1897, pp. 287-301. 



& Clarke and Hillebrand, cit. pp. 277-286. 



