I 76 STUDIES FOR STUDENTS 



True monosomatic chondri are confined almost exclusively to 

 the mineral chrysolite. They can be known by their simultane- 

 ous extinction in polarized light. Polysomatic chondri may be 

 made up of different minerals as well as different individuals 

 and may show more than one kind of structure, i. e., a chondrus 

 may be granular in one portion and fibrous in another. The fol- 

 lowing minerals are noted by Tschermak as forming chondri, their 

 relative abundance being in the order named : Chrysolite, bronzite 

 augite, plagioclase, glass, and nickel-iron. This order, it is to be 

 noted, is also that of the fusibility of the minerals, the most infusi- 

 ble and hence the earliest cooling mineral forming the most chon- 

 dri. Chrysolite chondri usually contain large quantities of glass 

 of a dark brown color. This may be arranged [a) in the form of 

 alternating layers, in which case a marked rod-like or lamelli- 

 form appearance is produced, or (#) may form a base in which 

 the mineral is developed porphyritically, or (V) may occur in the 

 center of a crystal, or (<sQ may form a network. Polysomatic 

 chondri of the latter sort are especially liable to be mistaken for 

 those of bronzite since they simulate the fibrous appearance of 

 the latter. Occasionally the crystallization may have proceeded 

 only far enough to produce skeletal or branching growths of the 

 mineral among the glass. Both monosomatic and polysomatic 

 chrysolite chondri may have the arrangement of a well-marked 

 rim about a spherical interior. This rim may, in the polyso- 

 matic chondri, be composed of many individuals. Such a rim 

 is often dark from a content of iron and troilite. Chromite, 

 either in minute grains or in dust-like aggregations, also forms a 

 common inclusion usually near the surface of the chondrus. The 

 quantity of opaque inclusions may be so great as to give the 

 chondrus a black color. Such chondri associated with those of 

 light color are to be found in the stones of Knyahinya, Mezo- 

 Madaras and others. The constituent minerals of such chondri 

 are chiefly chrysolite and bronzite. Bronzite chondri are usually 

 of a finely fibrous character. The fibers instead of radiating 

 from a center as do those of spherulites usually radiate from an 

 eccentric point. This eccentric arrangement constitutes one of 



