288 ORIGIN OF CRYSTALLINE ROCKS. [XIII. 



By far the greater number of cases on which this general 

 theory of pseudomorphism by a slow process of alteration in 

 minerals has been based are, as I shall endeavor to show, ex- 

 amples of the phenomenon of mineral envelopment, so well 

 studied by Delesse in his essay on Pseudomorphs,* and may 

 be considered under two heads : first, that of symmetrical 

 envelopment, in which one mineral species is so enclosed 

 within the other that the two appear to form a single crystal- 

 line individual. Examples of this are seen when prisms of 

 cyanite are surrounded by staurolite, or staurolite crystals com- 

 pletely enveloped in those of cyanite, the vertical axes of the 

 two prisms corresponding. Similar cases are seen in the en- 

 closure of a prism of red in an envelope of green tourmaline, 

 of allamte in epidote, and of various minerals of the pyrox- 

 ene group in one another. The occurrence of muscovite in 

 lepidolite, and of margarodite in lepidomelane, or the inverse, 

 are well-known examples, and, according to Scheerer, the crys- 

 tallization of serpentine around a nucleus of olivine is a similar 

 case. This phenomenon of symmetrical envelopment, as re- 

 marked by Delesse, shows itself with species which are gener- 

 ally isoinorphous or homoeomorphous, and of related chemical 

 composition. Allied to this is the repeated alternation of crys- 

 talline laminae of related species, as in perthite, the crystalline 

 cleavable masses of which consist of thin, alternating layers of 

 orthoclase and albite. 



Very unlike to the above are those cases of envelopment in 

 which no relations of crystalline symmetry nor of similar 

 chemical constitution can be traced. Examples of this kind 

 are seen in garnet crystals, the walls of which are shells, 

 sometimes no thicker than paper, enclosing, in different exam- 

 ples, crystalline carbonate of lime, epidote, chlorite, or quartz. 

 In like manner, crystalline shells of leucite enclose feldspar, 

 hollow prisms of tourmaline are filled with crystals of mica or 

 with hydrous peroxide of iron, and crystals of beryl with a 

 granular mixture of orthoclase and quartz, holding small crys- 

 tals of garnet and tourmaline, a composition identical with the 

 * Annales des Mines (5), XVI. 317-392. 



