PYEOPLASTIC EOCKS. 
107 
stricted to that form of greenstone which is inclosed within the 
veins of a fissure. It is commonly called a trap dyke. It is a 
stony and not a metalliferous vein. Fig. 20 represents a dyke 
intersecting two parallel veins of 
granite. The forms of the pyro- 
plastic rocks were determined by 
the condition under which they 
cooled. We can not always de¬ 
termine now what that condition 
was. It has been demonstrated 
that slow cooling restores to the 
mass the common characters of 
a rock, free from vitrification. So 
under certain circumstances the mass separates into columns of 
five or six sides. The tendency to the columnar condition is 
distinct, while the columns are often imperfect. Their terminal 
outline is visible, but the adhesion of their sides still remains. 
The columns are vertical, as at the Palisades of the Hudson, or 
they may be horizontal, as when enclosed between the walls of 
a fissure. 
Fig. 21 . 
a Sandstone, b Columnar Trap, c Trap injected between the strata of sandstone. 
Fig. 20. 
