94 



EOCKS FORMED TIIEOUGH IGNEOUS AGENCIES 



Chemical Composition. — The following analyses serve to show 

 the variations which are due mainly to the varying character of 

 the pyroxenic constituents: 



Chemical Composition op the Pyroxenites 



CONSTITtJBNTS 



Silica (Si02) . . . . 

 Alumina (AI2O3) . . 

 Chrome oxide (61203) 

 Ferric oxide (Fe20s) . 

 Ferrous oxide (FeO) . 

 Manganese (MnO) . . 

 Lime (CaO) . . . . 

 Magnesia (MgO) . . 

 Soda (Na20) . . . . 

 Potash (K2O) . . . 

 Water (H2O). . . . 

 Chlorine (CI) . . . 



60.80 % 



3.40 



0.32 



1.39 



8.U 



0.17 



12.31 



22.77 



Trace 



Trace 



0.62 



0.24 



100.03% 



II 





63.98 % 



1.32 



0.63 



1.41 



3.90 



0.21 

 16.47 

 22.59 



0.83 



55.14 % 



0.C6 



0.26 



3.48 



4.73 



0.03 



8.39 

 26.66 



0.30 



0.38 

 0.23 



100.24 % 



100.26 % 



I. Hypersthene-diallage rock: Johnnj Cake Eoad, Baltimore County, 

 Maryland. II. Hypersthene-diallage rock: Hebbville post-office, Baltimore 

 County, Maryland. III. Bronzite-diopside rock from near Webster, North 

 Carolina. 



Structure. — The py- 



roxenites are holocrystal- 

 line granular rocks, at 

 times evenly granular and 

 saccharoidal, or again 

 porphyritic, as in the 

 websterite from 

 Carolina. The 

 scopic structure 

 rock is shown in 



North 



miero- 



of this 



Fig. 6 



from the original draw- 

 ing by Dr. Williams. 



Colors. — The colors 

 are, as a rule, greenish or 

 bronze. 



Classification and No- 

 menclature. — The pyrox' 

 enites, it will be observed, differ from the peridotites only in the 



Pig. 6.- 



-Microstructure of websterite, Web- 

 ster, North Carolina. 



