314 ORIGIN OF CRYSTALLINE ROCKS. [XIII. 



Delesse begins his argument by remarking that since, in some cases, 

 a mineral is found to be surrounded by another clearly resulting 

 from its alteration (as, for example, anhydrite by gypsum), certain 

 mineralogists have supposed that wherever one mineral encloses 

 another there has been epigenesis or pseudomorphous alteration. 

 Such, he says, may sometimes be the case, but it is easy to see that 

 it is not so habitually. A crystallized mineral species frequently 

 includes a large and even a predominating portion of another, and 

 the combination is then considered by many as an example of 

 partial pseudomorphous alteration. In such instances, remarks 

 Delesse, the question arises whether we have to do with the 

 results of envelopment or of chemical alteration ; to resolve which 

 it becomes necessary to study carefully the problem of envelopment. 

 He then proceeds to show that the enveloped substance is, in some 

 cases, crystalline (and arranged either symmetrically or asymmetri- 

 cally with regard to the enveloping mass) ; while in other cases it is 

 amorphous, and enclosed like the sand-grains which predominate iu 

 the calcite crystals of Fontainebleau. The difficulty in deciding 

 whether we have to do with envelopment or with epigenesis increases 

 when the enveloped mineral becomes so abundant as to obscure the 

 enveloping species, or when it becomes mixed with it in so intimate 

 a manner as to seem one with the latter (se fondre insensiblement avec 

 lui). The proportions of the enveloped and the enveloping mineral, 

 we are told, may so far vary that the one or the other is no longer 

 recognizable. " As the forces which determine crystallization have 

 a great energy, the enveloping mineral is sometimes found in so 

 small a quantity as to be entirely masked by the enveloping species." 

 " When minerals have crystallized simultaneously, they have been 

 able to become associated with each other and to envelop each other 

 in all proportions" (loc. cit., pages 338, 339, 341, 353). 



Our author then proceeds to tell us that, having carefully stud in 1 

 in numerous specimens the supposed mica-pseudomorphs of iolite, 

 andalusite, cyanite, pyroxene, hornblende, etc., he* regards them as, 

 in all cases, examples of envelopment, and expresses the opinion 

 that we must omit from our lists a great number of the so-calK-l 

 pseudomorphous minerals, especially among the silicates. The final 

 result of the process of envelopment is, according to Delesse, this 

 to give rise to mixed mineral aggregates, owin^ their external 

 forms to the crystallizing energy of one of the constituents, which 

 may !" ] -resent in so small a quantity as to be completely obscured 

 by the other matter present. From this condition of things result 



