1917] Eaklc: Minerals Associated with Crystalline Limestone 333 



original isometric crystal with an octahedral shape such as periclase 

 would have had. The internal structure of the hrucite clearly indi- 

 cates that they have formed under great pressure, such as would be 

 produced by expansion within a confined space. In a change from the 

 anhydrous periclase to the hydrous brucite an increase in volume of 

 nearly two and one-half times that of the periclase is necessary under 

 normal conditions, and this great swelling has caused sufficient pres- 

 sure not only to produce the twisted and fibrous internal structure of 

 the pisolites, but also to curve slightly the twinning planes of the 

 calcite bordering the cavities. 



The names predazzite and pencatite were given to a similar brucite 

 limestone from Predazzo in the classic Monzoni district of Tyrol. 

 Specimens of the predazzite show the same compressed and strained 

 brucites in the white limestone, and they have generally been held to 

 be brucite derived from periclase, and some unaltered periclase has 

 been found. Lanacek 1 holds from a petrographic study of the predaz- 

 zite that the pisolites are hydromagnesite rather than brucite. Some 

 of the brucite of the Crestmore limestone is altered to hydromagnesite 

 as a later change but the pisolites are in the main brucite, as shown by 

 the analysis. 



Hydromagnesite. — Some of the brucite pisolites have altered by 

 weathering into earthy white material which qualitative tests prove 

 to be the hydrocarbonate of magnesia. It is evidently secondary from 

 brucite and not from any periclase direct. 



Chondrocyte. — This is the only mineral in the list of which there 

 is no well-authenticated proof of its existence in the quarries. Chon- 

 drodite and its characteristic associate, spinel, might be expected 

 among the products of metamorphism in the Commercial Rock quarry, 

 but apparently fluorine was absent, as tests of the spotted limestone 

 which suggest chondrodite failed to show its presence. Specimens of 

 chondrodite have been collected from the Colton limestone. Brucite 

 may form from chondrodite but it much more probable that periclase 

 was the original mineral of the Crestmore rock. 



Graphite. — The only place in the quarries where graphite is promi- 

 nent in the limestone is on the south end of Chino Hill, in close asso- 

 ciation with the brucite. It is so thickly disseminated through the 

 limestone as small flakes and scales that the rock is of a dark gray 

 color in consequence. The pisolites of brucite are often surrounded 

 by a border of black graphite, probably so oriented by the solutions 



i Min. u. petr. Mitth., vol. 12, pp. 429, 447, 1892. 



