20 AMERICAN MUSEUM GUIDE LEAFLETS 
or “rusting”’ in the ground and is called the rust crust. Almost without 
exception the aérolites 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, but sometimes it has high luster. 
A few meteorites possess a dark-gray crust, and some show crust only 
in patches. qr 
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 
aérolites which have lain long in the earth or it is much decomposed, 
as may be seen by examining Long Is!and 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. ‘o quickly is the crust formed that even the 
smallest members of a meteorite shower usualiy 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 origina! 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 piézogiyphs. ‘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 piézoglyphs 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 flight through the atmosphere. 
Without going more deeply into the subject in general we may now 
turn our attention to the Foyer collection. 
