VOLUME EFFECTS OF HYDRATION AND DEHYDRATION. 179 



be due to the pressure. However, iu the lower part of the zone of ana- 

 morphism the temperature is frequently higher than 110° C, and under 

 such circumstances both the pressure aud the temperature may work together 

 to produce dehydration. 



The statement that the volume is decreased by dehydration is only 

 true provided the separated water, or a large part of it, escapes; for the 

 volume of the hydrated solid is less than that of the residual solid plus the 

 separated water; therefore, if the water could not escape, pressure would 

 tend to preserve the combination. Hence, the fact that the reaction does 

 take place in the zone of anamorphism shows that there is sufficient 

 pressure not only to separate the combined water from the rocks, making 

 it free water, but to squeeze the free water from the rocks as one can 

 squeeze the water from a sponge. The effective pressure doing the work 

 is equal to the pressure of the adjacent rocks less the weight of an equal 

 column of water extending to the surface. Thus, under mass-static condi- 

 tions, if the rocks have a specific gravity of 2.7, the effective weight in 

 producing dehydration and driving out the free water at a depth of 3,300 

 meters is that of a column of material of this height with specific g-ravity 

 of 1.7. Under mass-mechanical conditions, where the pressure as a result 

 of thrust may be much greater than that due to weight, the effective 

 pressure tending to separate the combined water is much greater. Conse- 

 quently, under such conditions dehydration may occur at much less depth 

 than under mass-static conditions. (See pp. 766-768.) 



One or two minerals may be mentioned which illustrate the processes 

 of hydration and dehydration in the two physical-chemical zones. Near 

 the surface and to a considerable depth, under mass-static conditions, 

 limonite and other hydrated oxides of iron develop. Deeper down, and 

 especially in connection with mass-mechanical action, limonite is dehy- 

 drated, and hematite is produced. As another illustration may be men- 

 tioned the somewhat similar compounds, chlorite and biotite. Near the 

 surface and under quiescent geological conditions chlorite forms. Deep 

 below the surface, and especially under mass-mechanical conditions, biotite 

 ordinarily develops. This is nowhere better illustrated than in the Michi- 

 gamme formation in the Marquette district of the Lake Superior region, 

 where these two minerals directly replace each other under the law just 



