308 CLK LEIGH 
katamorphism, reach their maximum incoherency, when the reverse, 
or reascensional process, is initiated, and they begin to strengthen 
themselves. Their pore-space becomes lessened under their own 
weight or that of superior beds, and solutions drop their load in the 
interstices; the rock becomes cemented. The tendency toward 
cementation seems by observation to be at its maximum at or just 
below the level of ground-water. Van Hise defines the belt of cemen- 
tation as extending from the ground-water level to the lower limit 
of free movement of water; i. e., to the bottom of the zone of fracture. 
Cementation consists essentially of a consolidation and selective 
addition of materials in response to environmental demands for 
greater coherency. The principal chemical change is due to loss of 
water and to the proportions of cementing materials added. ‘The 
resulting rocks are shales, sandstones, quartzites, and limestones, 
with the same kinds of minerals as in the unconsolidated sediments. 
The source and nature of the cementing materials may be summar- 
ized roughly by comparing the materials available in solution from 
the weathering of igneous rocks with the requirements of the sedi- 
ments in the proportions above assumed. Observed percentage 
losses of elements—alumina being assumed constant, when averaged 
for cases of so-called ‘‘complete”’ weathering—show frequently about 
a 33 per cent. total loss of substances in solution when alumina is 
figured as constant. A hundred pounds of average igneous rocks 
therefore would contribute to solutions 33 pounds of material, of 
which 23 pounds is silica, 8 pounds calcium and magnesium carbon- 
ates, and the remainder soda, potassa, iron oxides, etc. ‘This mate- 
rial is delivered partly through the overground circulation ultimately 
to the ocean, and partly to the underground circulation, where it 
becomes available for cementation. After taking out enough of the 
materials in solution to afford the ocean salts and the chemical and 
organic sediments in their known proportion to the total, the over- 
whelming dominance of silica is apparent in the materials left for 
cementing purposes. Calculations show that of the net amount of 
23 pounds of material available for cementation in our average case, 
fully 20 pounds are silica. This is not out of accord with observed 
abundance of cements, when we take into account the mass of silica 
used in cementing shales, usually not considered. It is possible that, 
