OPHITES AND RELATED ROCKS, 23 



same bodies are present in hemithrenes from Ceylon, Aker 

 (Sodermanland in Sweden), and New Jersey*. The Vosges 

 is an additional locality, which we have only of late become 

 acquainted with. 



The question naturally suggests itself as to how the separated 

 aciculse in pectinated chrysolite, the arborescent configurations 

 in flocculite and malacolite, and the plates, lenticuloids, and 

 spheroids in serpentine have been produced. From the evidence, 

 so far adduced, our view will have been to some extent antici- 

 pated that it is by chemical reagents involving the removal of 

 serpentine or other mineral silicates, and their replacement by 

 calcite or other mineral carbonates. 



Chemical changes of the kind, known as pseudomorphism, 

 are not uncommon in the mineral kingdom. They may, for 

 our purpose, be arranged under two heads entire and partial, 

 The first consists of cases in which all the original constituents 

 of a mineral having been abstracted, are substituted by other 

 substances ; the second consists of cases in which the removal 

 of certain constituents of a mineral, and their replacement by 

 other substances, have taken place. 



As an example of the first, crystals are found in Cornwall 

 consisting of cassiterite or oxide of tin ; but, instead of repre- 

 senting the form proper to this mineral (viz. a modification of a 

 square prism), they occur under a false form the one that cha- 

 racterizes orthoclase, which is a silicate of alumina and potash: 

 in this instance an entire change of substance has taken place. 

 The second may be illustrated by selenite a hydrous sulphate 

 of lime, well known as crystallizing in right rhomboidal tabular 

 crystals ; but occasionally such solids are found consisting of 

 carbonate of lime, the basic constituent remaining, but the acid 

 and water eliminated, both having been replaced by carbonic 

 acid. Karstenite, an anhydrous sulphate of lime, when con- 

 verted into selenite, as it often is, is also a partial pseudomorph ; 

 though the change has been effected simply by the admission of 

 water f- 



* Geological Magazine, vol. x. no. 1, January 1873. 



f Because a few cases have occurred of what appears to be one mineral 

 enveloping another without change of crystalline form (e. g. prisms of anda- 

 lusite ferruginated into a substance having the composition of staurolite), 

 Dr. Sterry Hunt has " confidently affirmed that the obvious facts of envelop- 

 ment," which have led Delesse to limit pseudomorphism, as advocated by 



