Observations on Pseudomorphism. 89 



11. Pseudomorphous changes requiring an elevated temperature. 



Magnesian Pseudomorphs. — The magnesian pseudomorphs 

 appear to fall within this division. The magnesian minerals 

 constituting them are talc, including the variety steatite, saponite 

 or hydrous steatite, serpentine and chlorite. In the change of 

 feldspar to talc, magnesia by its affinity for silica, expels alumina,^ 

 and at the same time, the other elements of feldspar that cannot 

 enter into the new magnesian compound, pass off.* The same 

 takes place in the change of mica, kyanite, augite, &c. to talc. 

 The general theory of the process, corresponds then with that 

 stated on page 87, at bottom. 



Magnesia either in solution or in the state of vapor, has acted 

 upon the mineral and rocks that have undergone this change. 



The idea of magnesia in vapor is an assumption as yet unau- 

 thorized by chemical science, except as a theoretical possibility. 

 It may be convetiient to summon the hypothesis to our aid, for 

 the sake of an explanation ; but as far as facts have been ascer- 

 tained they stand against such a view. Many of the rocks so 

 altered to magnesian, contain water ; evincing thus that they 

 have not been subjected to that immeasurable degree of heat, 

 which the fancy might call up, for producing evaporation of this 

 most fixed and infusible of the earths. 



Magnesia compounds in solution are well known. Sea water 

 is such a solution, and one that has existed, as geological facts 

 testify, since geological changes commenced on the globe. And 

 are not these pseudomorphs some of the eflTects resulting through 

 its agency when under the influence of volcanic heat? The 

 view we would here sustain, is the same already presented in a 

 preceding volume of this Journal, namely, that the ocean waters 

 superincumbent, and penetrating the rocks, heated by igneous 

 action from below, have been the medium through which these 

 changes have been eflTected ; aided by additional magnesia and 

 other elements which the heat and pressure would enable them 

 to dissolve from the neighboring rocks. 



* Dr. Blum, speaking of this change, says that 6Mg (6 of magnesia) is substitu- 

 ted for K, Al, (1 of potash and alumina.) But is it not possible, considering the 

 affinities engaged, that under the causes in action, the potash and its silica to- 

 gether entered into combination with water, and were removed thus in solution, 

 and then ihe magnesia present displaced the alumina in the silicate of alumina 

 that remained ? See the remarks on the formation of kaolin, page 88. 



Vol. xLviii, No. 1.— Oct.-Dec. 1844. 12 



