J. D. Dana on Pseudomorphism. 261 



nal, namely, that the ocean waters, superincumbent, and 

 penetrating the rocks, heated by igneous action from below, 

 have been the medium through which those changes have 

 been effected ; aided by additional magnesia and other ele- 

 ments, which the heat and pressure would enable them to 

 dissolve from the neighbouring rocks. 



We cannot doubt, especially when we consider the vast 

 pressure of an ocean, that these waters, besides overlying, 

 fill up all open spaces or caverns between the layers of sub- 

 merged rocks ; and, probably, the whole substratum of the 

 ocean, when not too compact, is throughout permeated by 

 them. Submarine eruptions opening to the fires below, and 

 sometimes ejecting lavas, would distribute heat around, and 

 thus give the waters increased power in their action upon 

 rocks. This cause, therefore, is sufficient in its energy, and 

 sufficiently extended in its influence. It must have been in 

 action ever since the earth had an ocean and internal fires, 

 for it is a necessary effect of all submarine volcanic action. 

 "Wherever a trap-dike is to be found on the globe in a vein of 

 porphyry, there have been heated waters, permeating and 

 superincumbent, and under the pressure of an ocean intensely 

 heated. 



Additional magnesia would be furnished these waters by 

 the various augitic rocks, among which the traps are in- 

 cluded ; also by dolomites.* 



The action of heated waters is often indicated in the re- 

 gions of these pseudomorphs, by the rounded angles or pitted 

 surfaces of otherwise unaltered crystals. 



Other evidence that heated water has been the agent, is 

 found in the frequent change of quartz to steatite. Hot 

 magnesian waters alone could have effected the slow solution 

 of the silica, and the introduction of magnesia, that constitute 

 the change here indicated. Hot vapours, it is true, would 

 equally dissolye silica, but they would not afford the neces- 



* The interesting analyses of B. Silliman junior, have shewn that 

 the ocean's waters, through animal agency (the growth of corals), have 

 afforded a great part of the magnesia that is contained in dolomites. 



