584 Rogers — American Occurrence of Periclase. 



undoubtedly an alteration product of brucite, for in thin 

 sections it has the same structure as the brucite but can 

 be distinguished from the latter mineral by the fact that 

 the upper first-order interference colors are normal. Its 

 double refraction is about the same as that of brucite but 

 the indices of refraction are less than 1-55. 



One specimen showed the result of a still further altera- 

 tion, that of the hydromagnesite to a weak doubly refract- 

 ing, though probably amorphous, 9 hydrous magnesium 

 silicate which seems to be deweylite. It is a colorless to 

 pale green, compact mineral with an index of refraction 

 of 1-530 ± -003. 



The portions of limestone containing the deweylite 

 have evidently had a very complicated history. The fol- 

 lowing are the probable stages through which it has 

 passed: 



1. Sedimentary limestone. 



2. Dolomitic limestone. 



3. Dedolomitized limestone with periclase. 



4. Calcite-brucite rock. 



5. Calcite-hydromagnesite rock. 



6. Calcite-deweylite rock. 



This furnishes another illustration of the fact that the 

 minerals of a given rock or mineral deposit are formed 

 in stages one after another. Notwithstanding statements 

 to the contrary the contact-metamorphic deposits form 

 no exception to this general rule. The dedolomitization 

 and the consequent formation of periclase, chondrodite, 

 spinel, and pyrrhotite are the result of high-temperature 

 ascending solutions, presumably emanating from the 

 magma. The minerals just mentioned were probably 

 formed in stages also, but no evidence on this point was 

 obtained. 



The Crestmore occurrence is especially interesting* 

 because of the later after-effects of contact metamorph- 

 ism. 



The formation of brucite at the expense of periclase is 

 clearly later than the contact metamorphism and is prob- 

 ably due to a hypogene 10 process, for in the Riverside 



9 See paper by the writer on amorphous minerals, Jour. Geol., 25, pp. 

 515-541, 1917. 



10 This useful term was introduced by Eansome (U. S. Geol. Surv., Bull. 

 540, pp. 152-3, 1914) for minerals or ores formed by ascending solutions. 

 It and the corresponding term, supergene, used for minerals and ores 

 formed by descending solutions, avoid the ambiguity in the use of the 

 terms primary and secondary. 



