20 AMERICA. X MUSEUM GUIDE LEAFLETS 



or "rusting" in the ground and is called the rust crust. Almost without 

 exception the aerolites are covered with a crust, the appearance of which 

 varies according to the mineral composition of the mass. The crust is 

 almost always black and is usually dull, hut sometimes it has high luster. 

 A few meteorites possess a dark-gray crust, and some show crust only 

 in patches. 



The crust of the stone meteorites is glassy in character on account 

 of its being composed of silicates which have been cooled rapidly from 

 fusion. This glass, like glasses of volcanic origin, does not long resist 

 the atmospheric agents of decay, hence it is usually missing from those 

 aerolites which have lain long in the earth or it is much decomposed, 

 as may be seen by examining Long Island and Selma in the Foyer col- 

 lection. The crust varies in thickness on different parts of a meteorite 

 and often shows ridges and furrows which are due to friction with the 

 air. Frequently the ridges or furrows radiate from one or more centers 

 in such manner as to show which side of the mass was forward during 

 its flight through the air. So quickly is the crust formed that even the 

 smallest members of a meteorite shower usually possess a complete 

 crust. In the case of angular fragments the crust on the different sides 

 can usually be distinguished as "primary" or "secondary" according 

 to whether it was a part of the original exterior of the mass or was 

 formed upon the new surfaces exposed by the bursting of the meteorite. 



Another common surface characteristic of meteorites is an abundance 

 of shallow depressions or pittings, which on account of their form have 

 been called "thumb marks," or piezoglyphs. These pittings are so 

 shallow and superficial in character that exposure to the atmosphere 

 obscures or obliterates them in a comparatively short time. The rust- 

 ing of an iron meteorite may produce similar shallow depressions, as 

 will be seen from an examination of the surfaces of the great hollows 

 in "Willamette. The true piezoglyphs doubtless owe their origin to 

 several different causes, the most potent of which are unequal softening 

 of a mass due to varying chemical composition and rapidly changing 

 pressure and consequent erosion during Might through the atmosphere. 



Without going more deeply into the subject in general we may now 

 turn our attention to the Foyer collection. 



