2 
Field Museum of Natural History 
are probably smaller, as removal of the water tends 
to contract them. The pores of agate are too small 
to admit the absorption of large molecules like those 
of sugar or they admit them only with difficulty. This 
is shown by the fact that in the process of coloring 
agates black by means of a sugar solution and sul- 
phuric acid, from two to three weeks are required for 
the large sugar molecules to penetrate the agate, while 
only a few hours are necessary for the absorption of 
the smaller molecules of sulphuric acid. 
Owing, probably, to the fibrous structure and dens- 
ity of agate, it is tough and remarkably resistant to 
wear. It is somewhat harder than quartz. These qual- 
ities make it useful for many industrial purposes. 
Chemically, agate is pure silicon, the commonest com- 
ponent of the earth's crust. Of agates it may be said 
more truly than of other minerals, perhaps, that each 
individual is unique; one does not duplicate another. 
But although agates vary greatly in pattern and in 
color, yet they fall into certain natural groups based 
chiefly on pattern, which permit them to be given 
a certain mineralogical classification. 
The most common and characteristic form of 
agates is one which in cross section resembles an old- 
time bastion, a defensive work characterized by curv- 
ing contours and salient angles. Agates of this pattern 
are known as "fortification" agates (Plate I). Those 
agates in which the bands run approximately straight, 
are known as "ribbon" agates. This kind of band- 
ing is also characteristic of onyx and hence agates 
of this kind are sometimes known as "onyx" agates. 
Both patterns can sometimes be seen in the same 
agate (Plate IV). If the bands lie so close together 
that a ray of light in passing through them is broken 
up into prismatic colors, it distinguishes what are 
called "rainbow" agates. If the agate is irregularly 
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