4 ROCK-METAMORPHISM. 



Serpentine, which has never been found in a crystalline form 

 proper to itself, not unfrequently occurs in forms characteristic 

 of other minerals., as peridote, augite, hornblende, chondrodite, 

 phlogopite, garnet, diallage, spinel, feldspar, &c. In the case 

 of a mineral so prone to assume false forms as serpentine is, it 

 might be expected that some of the varieties and related species 

 would also display the like protean character. As cases in 

 point, loganite and picrosmine are pseudomorphs after horn- 

 blende ; while crystallized rensselaerite and pyrallolite occur in 

 the form of augite. 



The opinion that serpentine is in all cases a chemically 

 changed or secondary product involves the idea that its sub- 

 stance is a soluble compound. It is commonly stated, however, 

 that silicate of magnesia, the substance in question minus H 2 0, 

 is insoluble that it is one of "the most stable" compounds*. 

 Hence there are some who assume that serpentine cannot be 

 affected by ordinary chemical reagents. 



In denying the solubility of silicate of magnesia, it does not 

 follow that serpentine is also insoluble : besides, in this con- 

 nexion, there are some other considerations not to be over- 

 looked. 



1st. Serpentine is a hydrous silicate of magnesia, generally 

 adulterated with alumina, protoxide of iron, or other acces- 

 sories (see ante, p. 4), the presence of which make it liable to 

 chemical reactions. 



2nd f. It has existed under conditions of pressure and tempe- 

 rature capable of materially augmenting the potency of the 

 weakest dissolving agent that may have been in contact with 

 its constituents. 



Blum, " are adequate to explain all the cases of association upon which this 

 hypothesis of pseudomorphism by alteration has been based.'' Moreover, 

 although there may occur occasionally serpentine surrounding a nucleus of 

 peridote, such as led Scheerer to imagine that it " was a case of envelopment 

 of two isomorphous species " (serpentine has no proper crystalline form of its 

 own', therefore no other species can be isomorphous with it), it may be 

 regarded as certain, taking the consensus of opinion entertained by the 

 highest authorities, that the frequent entire replacement of the latter mineral 

 by the former is solely the result of chemical changes. 



* Bischof's ( Chemical and Physical Geology/ vol. ii. p. 113. 



1 We speak of the condition of serpentine before it became exposed at thg 

 earth's surface, 



