62 STUDIES FOR STUDENTS 



sufficient to permit accurate measurements of the crystal planes. 

 A curious feature of these crystals, however, and one which has as 

 yet received no adequate explanation is that they usually exhibit 

 a rounding of the solid angles and edges, giving an appearance 

 of a sphere on which facets have been cut. 



3. Stone 7iieteorites. — To what extent a primary crystalline 

 structure characterizes stone meteorites, is a point regarding 

 which, as has been said, no two observers are likely to agree. 

 The minerals of many meteorites occur in well crystallized form, 

 but whether they have crystallized in situ or are mere splinters 

 from previously existing masses is a disputed point in most 

 cases. The number of stone meteorites showing a holo-crystal- 

 line structure similar to that characterizing terrestrial rocks is 

 certainly small. Such as may be of this character are fine- 

 grained and resemble fine-grained basalts in their structure. 



Among minerals occurring in well defined crystal forms, 

 whatever their relation to the mass as a whole, enstatite, chryso- 

 lite, augite, and plagioclase are the most common and charac- 

 teristic. The crystals of these minerals usually have well-defined 

 boundaries and exhibit planes corresponding to those of terres- 

 trial minerals of the same kinds. Twinned individuals are com- 

 mon and a lamellar arrangement of inclusions is sometimes seen. 

 There is a complete absence, however, of layers of growth or of 

 zonal structure so common in the minerals of volcanic terres- 

 trial rocks. The crystal individuals often contain large quanti- 

 ties of glass and often present a highly fissile structure. Another 

 remarkable feature is a complete absence of fluid inclusions. 

 Gas pores while occasionally to be seen are exceedingly rare. 

 The latter fact, it may be remarked, furnishes a strong argument 

 against any theory which regards meteorites as having been 

 formed directly from vapors. 



CRYPTOCRYSTALLINE AND AMORPHOUS STRUCTURES 



I . Iron meteorites. — The term amorphous irons or ataxites is 

 usually used to designate iron meteorites which give no Widman- 

 statten figures on etching. Such irons are few in number and the 



