282 ELEMENTARY GEOLOGY 
thousand feet may have become so warm, and so fully expanded, 
that when they were later cooled by the removal of the blanket 
of overlying rock, their contraction caused them to break by 
jointing. ‘These rocks were also filled with water; and when 
they came near the surface, and some of this was drained out, 
there was a second possible cause for a decrease in bulk and 
consequent contraction.’ It is not impossible that in different 
places all these causes have been at work to produce joint 
planes; but the last two seem to be of the least importance. 
Fie. 162. 
Joint planes in granite quarry in Missouri. 
Regularity of the Plane. The joint planes cut the 
rock in perfectly straight courses, quite unlike the way 
in which rocks would break at the surface. There 
are no irregular and jagged edges, but perfectly smooth 
faces, the crack sometimes clipping off the ends of 
1JIn a clay bank the drying by loss of water often produces irregular 
jointing. 
