ORIGIN AND DEVELOPMENT OP ORE-DEPOSITS. 349 



carbonate in Northamptonshire, or those of the north of Spain 

 near Bilbao. 



Pseudomorphic changes, using the term in its more limited 

 sense, may be conveniently classed as follows : — 



A. Gain of components. 



1. Simple hydration, as when anhydrite is converted 

 into gypsum, or muscovite into damourite. 



2. Simple oxidation, as when magnetite is converted 

 into hematite, or native copper into cuprite. 



3. Addition of a compound radule, as when cuprite is 



converted into malachite. 



B. Loss of components. 



4. Simple loss of a constituent, volatile or soluble, as 

 when cuprite occurs as metallic copper, or argentite 

 as metallic silver. 



c. Substitution. As when galena is converted into cerussite, 

 or pyromorphite into galena, or chalybite into hematite. 

 The next three are pseudomorphs in quite a different sense. 



D. Infiltration into a cavity formerly occupied by another 



substance. In this case, the form but not the structure 

 will be preserved, as in the ease of the Wheal Coates 

 pseudomorphs of tin after orthoclase.* 



E. Infiltration of organic forms, as the cassiterite in form of 



cancellated horn structure of the chalcedonic " Beekites " 



of Torquay. 

 E. Pseudomorphism ef dimorphous substances. This is merely 



a molecular re-arrangement, as when calcite with its 



characteristic cleavage appears in form of aragonite or of 



stalactites. 

 All or nearly all of these modes of pseudomorphism are met 

 with in the mining region of the West of England, f 



* See the author's " Handbook to the Mineralogy of Cornwall and Devon " 

 article Pseudomorphs, Truro, 1876. 



f At Wheal Coates, in St. Agnes, fine crystals of oxide of tin replacing 

 felspar were found many years since in great quantities, in all stages from nearly 

 pure felspar to nearly pure peroxide of tin. They were not, therefore, casts of tin 

 in shape of felspar, but true replacements. See Tweedy, R.I.C. Similar 

 occurrences, abundant, but owing to conditions of main mass of elvan not 

 isolated, were found at Terras, Belowda, Castle-an-Dinas, &c. 



