178 J. Barrel! — Relations of Subjacent Igneous 



10 per cent by weight of combined water wbicli was not 

 in them at their origin and must have been supplied on 

 a large scale and with a penetrative power which carried 

 it to all parts of the rock mass when undergoing altera- 

 tion. 



Development of Anhydrous or Hydrous Silicates. 



In his classic work on metamorphism, Van Hise devel- 

 oped as an important subject that of changes of volume. 

 But he placed the emphasis upon meteoric waters as the 

 especial crystallizing agent of all anamorphic as well as 

 katamorphic changes. On that basis, and on the assump- 

 tion that water and. gases could freely escape, the pres- 

 sure resulting from depth was shown to tend toward the 

 elimination of both occluded and combined water. The 

 evidence shows this to be really effective in the diagenesis 

 of the sedimentary rocks, resulting in a partial dehydra- 

 tion of kaolin and opal, and often in a complete 

 dehydration of ferric oxide. 



Let us look, however, at the pressure effects of deep- 

 seated magmatic waters. They are driven into the rocks 

 by the excess pressure from below. A complete escape 

 above is slow and difficult because of the non-porous and 

 unfissured state of the rocks at great depths. A. C. 

 Lane^^ has shown that under such circumstances the 

 gases may be able to rend the rocks and create a condi- 

 tion of the zone of fracture deep within the zone of flow. 

 In so far, however, as the water as gas or liquid is under 

 greater pressure than the surrounding rocks, it will tend 

 to enter into the form of hydrates, since the hydrated 

 minerals in general occupy less volume than the equiv- 

 alent anhydrous minerals plus the water in uncombined 

 form. For magmatic waters, therefore, there is opposi- 

 tion between the factors of temperature and pressure. 

 Hydration will, consequently, take place at distinctly 

 higher tem]3eratures than for circulating waters of 

 surface origin. 



The importance of this principle can be illustrated by a 

 numerical example based on the extreme or limiting 

 supposition that the water and gases involved as chemical 

 constituents in the reaction can not escape but require 

 space for their existence in the same measure as do the 

 solid constituents. Assume that the density of the water 



"Bull. Geol. Soc. America, 5, 259-280, 1894. 



