586 Rogers — American Occurrence of Periclase. 



of Big Cottonwood Canyon, twenty-five miles southeast 

 of Salt Lake City, Utah, has been studied by the writer. 

 Calcite and brucite are practically the only minerals 

 present. The brucite occurs in subhedral, more or less 

 rounded, equant aggregates which have exactly the same 

 structure as the Riverside and Crestmore specimens and 

 are doubtless pseudomorphous after original periclase. 

 Fig. 2 shows the general character of the rock in thin 

 sections which is almost identical with a predazzite from 

 Fassathal, Tyrol. 



5. Other Occurrences of Calcite-Brueite Rocks in the United 



States. 



Emmons and Calkins 11 report a crystalline limestone 

 from the Phillipsburg quadrangle, Montana, which con- 

 tains brucite and which they say is pseudomorphous after 

 some unidentified mineral. The original mineral was 

 probably periclase as they speak of brucite occurring in 

 i ' aggregates of microscopic fibrous or scaly individuals ' ' 

 and dedolomitized rocks are prominent in the region. 



Summary and Conclusions. 



1. The first recorded American occurrence of periclase 

 is at Riverside, California, in a crystalline limestone. 



2. Calcite-brucite rocks (the so-called predazzite) are 

 formed from periclase-bearing limestones by the altera- 

 tion of periclase to brucite. 



3. The hydration of periclase to form brucite is prob- 

 ably brought about by hydrothermal ascending solutions 

 in spite of the fact that brucite contains about thirty- one 

 per cent of water. 



4. At a later stage the brucite may be converted into 

 hydromagnesite, a mineral similar to brucite in general 

 characters and one that may easily be mistaken for 

 brucite. 



5. In crystalline limestones as in other rocks and 

 mineral deposits in general the minerals are formed in 

 stages one after another. 



Stanford University, California. 



11 U. S. Geol. Surv., Prof. Paper 78, p. 157, 1913. 



