GEOLOGY OF MOUNT MARCY 



19 



as ihey might at first appear. The limestones are so often molded 

 like dough that they might easily convey to one not aware of their 

 plasticity the impression that they were intrusive. The phenomena 

 of contact metamorphism cover all the other features and have 

 become much better understood since Professor Emmons wrote. 



The diopside is of a beautiful yellowish green in section and some- 

 what darker green in the hand specimen. As is so often the case with 

 silicates included in limestone, it is rounded as if corroded. Only 

 rarely can the semblance of a crystal face be detected. Coccolite 

 would be the most appropriate name for it, and by. this term it has 

 been usually described. The grains range up to one-fourth of an 

 inch (6 mm) in diameter and exhibit the characteristic cleavage (see 

 plate 7). They constitute about 60 per cent of. the mass of some 

 specimens of the rock. With them are associated a few scattered 

 garnets of smaller size, black in the hand specimen, yellowish brown 

 in thin section, and of rounded outlines. In the slide they are 

 included in the diopside and have an isotropic core surrounded by 

 a doubly refracting zone. The remainder of the rock is calcite. 



Through the kindness of Dr G. S. Rogers, at the time instructor 

 in mineralogy at Columbia University, an analysis of the diopside 

 has been prepared. The crystals were separated from the calcite 

 and were washed in dilute HCl to remove the last traces of the 

 carbonate. 



' 



Molec. Rat 



OS 



SiO= 5 



1.84 





864 



) 866 



Ti02 



.20 





2 



AI.O3 



7.61 





• 74 



^ 8. 1 



Fe.O. 



1-75 





.10 



84 



FeO 



1.02 





14 



^ r 



MnO . 12 



MgO 1 1 . 39 





28s 



> 760 ^ 



CaO 25.80 





460 



H.O-t- 



• 43 





24 





100.16 





When recast we obtain 











CaOSiO. 







53.36 per cent 





MgOSiO^ 







20.10 





FeOSiO^ 







1.98 





MgO.AUOs 



.SiO 





14^95 





MgO.Fe20. 



.SiO 



2 



2.60 





Quartz 







6.24 





SiO^ 







.20 





H.O 







• 43 



840 



864 



99.86 



Thus about three-fourths of the mineral is the diopside molecule. 

 2 



