36 ELEMENTARY GEOLOGY 
lack of room for development, which will not allow 
the evolution of perfect form. Hence the perfect 
crystal is rare; but while under these conditions the 
boundaries cannot be present, the imternal structure 
is that of the crystal, and the mineral is truly erystal- 
line (Fig. 4). That is to say, the molecules of the _ 
elements unite according 
to the regular laws pro- 
ducing the crystal struc- 
ture, but cannot attain 
perfect form because the 
conditions will not per- 
mit. It is as if we 
planted two trees, one 
in the open air, the 
other in an insufficient 
space: the one would 
Fie. 4. be erect and perfect, the 
Calcite, a crystalline mineral without 
a) “ ay . jf P= 
crystal form. Cleavage faces shown. other i regula and sei 
tracted, but each would 
have the structure of a tree. So, while the crystal is 
one of the most imteresting and beautiful things in 
nature, and is of importance to the mineralogist, it con- 
cerns the geologist so little that in this book we may 
disregard it. 
In our elementary study of the earth, we must con- 
fine ourselves, also, to only the most important of the 
