Rogers — American Occurrence of Periclase. 585 



occurrence secondary magnetite occurs in the hrucite 

 aggregates and seems to have been formed in part at 

 least from the iron of the original periclase. Magnetite 

 is a typical hypogene mineral usually formed at compara- 

 tively high temperatures and in no known occurrence 

 does it appear to have been formed from descending 

 meteoric waters. 



The formation of a mineral containing such a large 



Fig. 2. 



Fig. 2. Thin section ( x 80) of calcite-brucite rock (6 = brucite, c = calcite). 

 Mountain Lake mine, Utah. 



percentage of water as brucite (ELO = 30-8 per cent) by 

 ascending solutions is unusual but not improbable. 



Whether the hydromagnesite is a hypogene or super- 

 gene mineral is difficult to say. The problem of deter- 

 mining the end of the hypogene stages and the beginning 

 of the supergene is an important one, but very difficult 

 in the present state of our knowledge. All possible data 

 bearing on this problem should be recorded in every 

 description of any kind of a mineral occurrence whether 

 ore-minerals are present or not. It seems reasonable to 

 regard the deweylite as a supergene mineral. 



4. Calcite-Briicite Rock from the Mountain Lake Mine, near 

 Salt Lake City, Utah. 



Another brucite-bearing crystalline limestone from a 

 contact zone at the Mountain Lake mine, near the head 



