Rogers — American Occurrence of Periclase. 581 



Art. XXV. — An American Occurrence of Periclase and 

 its Bearing on the Origin and History of Calcite-B rue- 

 it e Rocks; by Austin F. E ogees. 



1. The Occurrence of Periclase at Riverside, California. 



The rare magnesium-oxide mineral, periclase, not pre- 

 viously known from this country, has recently been recog- 

 nized in a specimen of crystalline limestone kindly sent 

 to me by Mr. Lazard Calm of Colorado Springs. This 

 specimen was found by Mr. Calm at the City quarry in 

 Riverside, California. The limestone is a medium- 



Ftg. 1. 



Fig. 1. Thin section ( x 18) showing core of periclase within brucite. 

 Eiverside, California. 



grained rock consisting largely of calcite and dark gray 

 to brown spots of brucite from 1 to 3 mm. in diameter. 

 The periclase occurs as cores of 1 mm. maximum size 

 within some of the brucite spots as shown in fig. 1. The 

 periclase is a colorless mineral with jDerfect cubic cleav- 

 age. Crushed fragments are square or rectangular in 

 shape, dark between crossed nicols and have an index of 

 refraction greater than 1-740. It is soluble in aqua regia 

 and the solution gives a good test for magnesium and a 

 slight test for iron. 



Under the microscope the brucite proves to be an 

 aggregate made up of concentric layers with a fibrous 

 structure, the fibers having an elongation parallel to the 

 faster ray. An attempt is made to illustrate this struc- 

 ture in fig. 1. The brucite shows anomalous interference 



