56! 



REPORT OF NATIONAL MUSEUM, 1890. 



Mineral composition. — The essential constituents are one or more 

 minerals of the pyroxene group, either orthorhombic or monoclinic. 

 Accessory minerals are not abundant and limited mainly to the iron 

 ores and minerals of the hornblende or mica groups. 



Chemical composition. — The following analyses serve to show the 

 variations which are due mainly to the varying character of the pyr- 

 oxenic constituents : 



Silica 



.Alumina 



Chrome oxide 

 Ferric oxide.. 

 Ferrous oxide 

 MangaDeso .. 



Lime 



Manganesia.. 



Soda 



Potash 



Water 



Chlorine 



Total .. 



50.80 



3.40 



0.32 



1.39 



8.11 



0.17 



12.31 



22. 77 



Trace... 



Trace... 



0.52 



0.24 



100. 03 



II. 



53.98 

 1.32 

 0.53 

 1.41 

 3.90 

 0.21 

 15.47 

 22.59 



III. 



55.14 

 0.66 

 0.25 

 3.48 

 4.73 

 0.03 

 8 39 



20.66 

 0.30 



0.38 

 0.23 



100.25 



(1) Hypersthenediallage rock, Johnny Cake road, Baltimore County, 

 Maryland ; (II) hyperstheue diallage rock, Hebbville post-office, Balti- 

 more County, Maryland, and 

 (III) bronzite diopside rock, 

 from near Webster, North Car- 

 olina. 



Structure. — The pyroxenites 

 are noncrystalline granular 

 rocks, at times evenly granular 

 and saccharoidal, or again por- 

 phyritic, as in the websterite 

 from North Carolina (speci- 

 mens 38832 and 17927). The 

 microscopic structure of this 

 rock is shown in Fig. 98 from 

 the original drawing by Dr. 

 Williams. 



Colors. — The colors are, as a 

 rule, greenish or bronze. 

 Classification and nomenclature. — The pyroxenites, it will be observed, 

 differ from the peridotites only in the lack of olivine. Following Dr. 

 Williams's nomenclature, we have the varieties Dillagite, Brouzitite, 

 and Hypersthenite, according as the mineral dillage, bronzite, or hypers- 

 theue forms the essential constituent. Websterite is the name for the 

 enstatite-dillage variety, such as occurs near Webster, North Carolina 



Fig. 98. 



MlCUO-STHUCTURK OF PYKOXENITE. 



