Hunt.] 334 [March 4, 



dominating quantities of foreign matter. From the analysis of such 

 mixtures chemists have concluded an epigenic change, more or less 

 complete, of the crystal. A good example of this is seen in certain 

 orthoclase crystals which include a large admixture of cassiterite. 

 Besides these cases of asymmetrical envelopment, we have those of 

 symmetrical envelopment. The occurrence of red tourmalines en- 

 closed in green ones, and of muscovite in margarodite, are clearly 

 cases of the deposition of one mineral upon another in the slow 

 growth which takes place in veinstones. In these also, by a process 

 which is often seen in the crystallization of certain species, such as 

 saltpetre, hollow or skeleton crystals are formed. The crystals of 

 tourmaline from Paris in Maine are in many cases prismatic tubes, 

 which are either empty or filled with mica. In like manner, crystals 

 of beryl are found which are filled with orthoclase, and similar shells 

 of zircon and of galena enclose calcite. The envelope is often ex- 

 ceedingly thin, and in the case of some species may be removed by 

 such selective solution as often takes place in mineral veins, leaving 

 the enclosed mineral with the form of the mould. 



The greater part of all the examples of replacement and envelop- 

 ment known hare been described as cases of pseudomorphism by 

 epigenesis, and the advocates of the doctrine of transmutation have 

 not hesitated to assert, upon this supposed evidence, the conversion 

 of almost every mineral species into some other, and to extend this 

 view to rock-masses, declaring that the great part of all the so-called 

 metamorphic or crystalline rocks are the results of an epigenic pro- 

 cess ; a doctrine which has been embodied in the dictum of Prof 

 Dana, that " regional metamorphism is pseudomorphism on a broad 

 scale." For an illustration of this doctrine and its legitimate conse- 

 quences, see the speaker's presidential address before the American 

 Association for the Advancement of Science in 1871, and in the Amer- 

 ican Journal of Science for July, 1872, his reply to Dana's criticisms 

 thereon. 



While the advocates of this doctrine maintain that a mass of 

 granite or diorite maybe converted into serpentine or limestone, and 

 that a limestone may he changed into granite or gneiss, which may, 

 in its turn, become serpentine, it is evident that it makes little differ- 

 ence what mineral species is taken for the starting point. Dr. Genth, 

 unlike his predecessors, takes his departure from corundum, and from 

 various facts in the association and envelopment of minerals found 

 accompanying it, is led to conclude that there have been formed 



