332 University of California Publications in Geology [Vol.10 



and small amounts of metallic sulphides. The beautiful blue calcite 

 occurring only in the north hill is a recrystallization by these solutions, 

 and some of the minerals, like the brucite, apophyllite, okenite, crest- 

 moreite, prehnite, laumontite, etc., have been formed by the action of 

 such solutions on pre-existing minerals. 



Zones or bands of contact metamorphic silicates separate the white 

 crystalline marble from the dikes of monzonite and pegmatite, and 

 some of the material is a compact massive and inseparable mixture. 

 Vesuvianite, garnet, and wollastonite are the abundant silicates of 

 Sky Blue Hill. Plate 22 shows three views of the Commercial Rock 

 quarry on this hill. 



Description op the Minerals 



MINERALS DISSEMINATED IN THE WHITE LIMESTONE 

 OF CIIINO HILL 



Brucite. — The magnesia hydrate is not disseminated through the 

 limestone as plates or scales but occurs wholly in rounded, pisolitic 

 bodies. The pisolites are generally quite prominent because of their 

 gray, yellow or pink color in contrast to the white calcite. Magnified 

 sections show them to be composed of a congeries of thin, overlapping, 

 curved plates and threads, more or less concentrically arranged, and 

 with a cross-fibered structure. This internal structure indicates great 

 strain and pressure in their formation, and little can be made of them 

 optically. The general interference colors are low, but their com- 

 pressed, interwoven fibrous structure prevents extinction. An analyses 

 of the pisolites gave : 



Some of the brucite is deeply colored by ferric oxide, which imparts a 

 yellow and red spotted appearance to the white limestone. Near the 

 surface of the hill in the Commercial Rock quarry, where the brucite 

 ha.s been weathered out, the limestone is cellular, with the cavities 

 lined with the yellow or red oxide. 



Periclase is assumed to be the original mineral which has changed 

 to brucite by simple hydration. No periclase has been observed and 

 no other mineral occurs in the limestone from which brucite could 

 be derived. The very regular form of the pisolites suggests an 



MgO 

 Fe,0 3 

 ILO 



67.48% 



0.55 

 31.73 



99.76 



G = 2.39 



