60 ELEMENTARY GEOLOGY 
large crystals to grow; but if we allow it to cool 
slowly, and then to slowly evaporate, we may build up 
some very large and nearly perfect crystals. So it is 
in lavas: if they cool quickly, the elements have 
neither time nor opportunity to come together, and 
the crystals are either minute or absent; but in case 
Fia. 19. 
Photograph of black obsidian rock, showing glassy or conchoidal fracture. 
of slow cooling, the crystals become larger, though 
rarely perfect in form, because they have not the 
room in which to freely develop. 
Hence, according to the rate of cooling, and the con- 
sequent perfection of mineral growth, there is every 
gradation from a glass without any crystals, to fine- 
