1 91 7] Eaklc: Minerals Associated ivith Crystalline Limestone 331 



SiO, 

 E 2 3 

 CaO 

 MgO 



Loss 



4.50% 



1.20 

 50.78 



2.50 

 40.60 



4.68% 



1.24 

 50.85 



2.25 

 40.36 



4.26% 



1.66 

 50.00 



2.54 

 41.54 



5.24% 



1.40 

 50.33 



2.60 

 40.40 



99.58 



99.38 



100.00 



99.97 



Analyses show that about two and one-half per cent of magnesia is 

 the average amount of that oxide in the white limestone or marble, and 

 it is therefore not very dolomitic. Associated with, and merging into 

 this rock are masses of similar white crystalline limestone containing 

 much magnesia, as the mineral brucite, disseminated through it in 

 pisolitic-shaped inclusions. The brucite rock apparently overlies the 

 other limestone, and is seen very prominently on the south end of 

 Chino Hill and near the surface in the Commercial Rock quarry on 

 Sky Blue Hill. It may be a remnant of a separate and distinct bed 

 of magnesian limestone, but the amount of the brucite varies consider- 

 ably and it has evidently been formed from some other magnesium 

 mineral, whose origin was due to contact metamorphism, and the 

 source of the magnesia might have been in the solutions accompanying 

 such metamorphism. 



Metamorphism of the Limestone. — The two hills are utterly dis- 

 similar in the effects of metamorphism. The limestone of Chino Hill 

 was converted into a white marble with very little development of 

 included metamorphic minerals. It does not appear to have suffered 

 successive metamorphism and recrystallization as would be induced by 

 later injections of igneous rock, and the original intrusion of the 

 granodiorite accounts for its simple metamorphism. At the south end 

 of the quarry the rock grades into a brucite-graphite limestone ; and 

 the brucite and graphite are so thickly disseminated as to warrant the 

 assumption that the original rock was a highly carbonaceous, mag- 

 nesia-bearing limestone which, by metamorphism, became converted 

 into a mixture of calcite, periclase, and graphite, the periclase subse- 

 quently altering to brucite. 



The Sky Blue Hill portion of the limestone capping was subjected 

 to later and more intensified metamorphism by intrusions of the 

 quartz-monzonite and pegmatite, and by the hydrothermal action of 

 the silicated-carbonated solutions accompanying or following these 

 intrusions. Practically all of the minerals developed are products of 

 hydrothermal metamorphism. These solutions also carried phosphates 



