J. D. Dana on Pseudomorphism. 263 



Calc-Spar to Bitter-Spar. — This change comes under the 

 same head with those first explained. A change of this kind 

 does not require a complete solution of the lime ; for late in- 

 vestigations have shewn that molecular attractions take place 

 at higher temperatures, without solution of all the elements 

 concerned in the change. 



Carbonate of Lead to Minium. — Minium is easily produced 

 from carbonate of lead by means of heat. 



Andalusite, Wernerite, and Tourmaline to Mica. — These 

 changes must have required the high temperature that crys- 

 tallized the mica in the metamorphic rocks. 



Auqite to Opal. — Dr Blum suggests that this change is 

 probably effected by means of the sulphur vapours of the vol- 

 cano decomposing the volcanic rock. The writer has seen 

 siliceous incrustations produced in this way at the sulphur 

 beds of Kilauea, Sandwich Islands, where the basaltic lavas 

 are gradually crumbling down through this action, to a yel- 

 low earth. The sulphuric acid removing the lime of the 

 augite forms sulphate of lime, which is found in snowy fibrous 

 seams and incrustations. 



We do not attempt to offer explanations of all the several 

 instances of pseudomorphs. A more particular examination 

 of the various localities, the associate minerals, and their 

 situations in the rocks, is necessary before this can be done- 



The principal obstacle in the way of chemically investi- 

 gating and imitating these processes in nature, lies in these 

 two facts : First, That it is difficult, by laboratory expei'i- 

 ments, to appreciate the effect of agents acting slowly for a 

 long period of time, under circumstances of temperature and 

 electric forces beyond our cognizance ; Second, vfe cannot 

 know fully the exact conditions and composition of waters 

 intensely heated under pressure, and rendered by this means 

 capable of dissolving many refractory minerals. 



the mystery that hangs around this subject. The fact, also, that these 

 crystallizations have taken place without evidence of the intense heat 

 necessary for fusion, is fully explained ; for, as Keilhau states, crystal- 

 lization has been lately shewn by many facts to tate place without fu- 

 sion, or even softening, yet not, as this author would urge, without somo 

 increase of temperature. 



