1918.] The N.Z. Journal of Science and Technology, 
57 
by atmospheric weathering, thereby liberating silica, by the redeposition 
of which the particles are cemented together. This silicification is par¬ 
ticularly prominent on the surface and along the joint-planes, and a hard 
crust is formed which protects the softer material within. For this reason, 
but also in part on account of the general porosity of the rocks, the tuffs 
do not readily produce a subdued topography, but form vertical cliffs along 
the river-valleys. 
