APPLICATION OF PHYSICO-CHEMICAL PRINCIPLES. 281 
pressure is equal to the weight of the superjacent rocks, less the weight 
of an equal column of water. Thus 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 gravity of 1.7. This would be the case under 
mass static conditions; under mass dynamic 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 water would be much 
greater; consequently, under such conditions, dehydration may occur at 
much less depth than under mass static conditions (see pages 309, 310). 
One or two minerals may be mentioned which illustrate the processes 
of hydration and dehydration in the two physico-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 dynamic action, hematite is fre- 
quently produced by dehydration of the hydrated oxide. As another 
instance may be mentioned the somewhat similar compounds, chlorite 
and biotite. Near the surface and under quiescent mass conditions 
chlorite forms. Deep below the surface, and especially under mass dy- 
namic conditions, biotite ordinarily develops. This is nowhere better 
illustrated than in the Michigamme formation,* in the Marquette district 
of the Lake Superior region, where these two minerals directly replace 
each other under the law just stated. 
A second important reaction separating the outer crust of the earth 
into two physico-chemical zones is the mutual replacement of oxygen 
and sulphur. In the upper zone oxygen replaces sulphur, and at the 
same time may largely oxidize that element. This results in great 
liberation of heat, and also in expansion of the volume of the solid com- 
pound. Oxidation may take place without replacing another element, 
as when iron protoxide is changed into iron sesquioxide with expansion 
of volume and liberation of heat. 
In the lower zone sulphur replaces oxygen with condensation and 
with great absorption of heat. In the case of the most common replace- 
ment, that of the oxygen united with iron by sulphur, the absorption of 
heat is very great indeed, asis shown by the following reactions: 
Fe+ 0+ H,O= Fe O,H, + 683 K. 
Fe +S + nH,0 = FeS nH,0 + 238 K. 
K is the large calorie and is equal to 1,000 small calories. From these 
equations it is apparent that in the replacement of the O of FeO by S 
*The Marquette iron-bearing district of Michigan, by C. R. Van Hise, W.S. Bayley, and H. L. 
Smyth: Mon. U.S. Geol. Survey, no. xxviii, 1897, pp. 452-459. 
