290 ORIGIN OF CRYSTALLINE ROCKS. [XIII. 







The above characteristic examples of symmetrical and asym- 

 metrical envelopment are cited from a great number of others 

 which might have been mentioned. Very many of these are 

 by the pseudomorphists regarded as results of partial altera- 

 tion. Thus, in the case of associated crystals of andalusite 

 and cyanite, Bischof does not hesitate to maintain the deriva- 

 tion from andalusite of the latter species by an elimination of 

 quartz ; more than this, as the andalusite in question occurs in 

 a granite-like rock, he suggests that itself is a product of the 

 alteration of orthoclase. In like manner the mica, which in 

 some cases coats tourmaline, and in others fills hollow prisms 

 of this mineral, is supposed to result from a subsequent altera- 

 tion of crystallized tourmaline. So in the case of shells of 

 leucite filled with feldspar, or of garnet enclosing epidote, or 

 chlorite, or quartz, a similar transformation of the interior is 

 supposed to have been mysteriously effected, while the external 

 portion of the crystal remains intact. Again, the aggregates, 

 of cassiterile, quartz, and orthoclase, having the form of the lat- 

 ter, are, by Bischof and his school, looked upon as results of 

 a partial alteration of previously formed orthoclase crystals. 

 It needed only to extend this view to the crystals of calcite 

 enclosing sand-grains, and regard these as the result of a par- 

 tial alteration of the carbonate of lime. There is absolutely 

 no proof that these hard crystalline substances can undergo the 

 changes supposed, or can be absorbed and modified like the 

 tissues of a living organism. It may, moreover, be confidently 

 affirmed that the obvious facts of envelopment are adequate to 

 explain all the cases of association upon which this hypothesis 

 of pseudomorphism by alteration has been based. Why the 

 change should extend to some parts of a crystal and not to 

 others, why in some cases the exterior of the crystal is altered, 

 while in others the centre alone is removed and replaced by a 

 different material, are questions which the advocates of this 

 fanciful hypothesis have not explained. As taught by Blum 

 and Bischof, however, these views of the alteration of mineral 

 species have not only been generally accepted, but have formed 

 the basis of the generally received theory of rock-metamor- 

 phism. 



