618 A TREATISE ON METAMORPHISM. 



abundant colloidal silicic acid and lesser proportions of the rarer bases and 

 acids, such as manganese, hydrofluoric, and boric. In the following section 

 on metasomatism it will be shown that during the alterations of the materials 

 of the belt of cementation equal or greater amounts of these compounds are 

 added to the solutions. A portion of the great amount of material which 

 joins the sea of ground water from the belt of weathering and from the 

 alterations of the minerals of the belt of cementation is there precipitated 

 as minerals' and the rocks are cemented. Hence the name, "belt of 

 cementation." 



It will be remembered that the openings in rocks comprise those 

 between the mineral particles, and especially the grains of sediments, the 

 vacuoles of igneous rocks, the regular openings of fissility, joints, and 

 faults, and the irregular openings from those of the fractures of the 

 individual mineral particles to those of breccias. Further, it will be 

 recalled that the amount of openings varies from a small fraction of 1 per 

 cent to 75 per cent, that in the mechanical sediments and the porous lavas 

 a pore space of 20 to 40 per cent is common, and that the quantitative 

 importance of the fault, joint, and fissile openings is great. (See pp. 124- 

 131.) Therefore the amount of material which is required to fill the open- 

 ings of great formations, thousands of meters in thickness and extending 

 over areas of hundreds or thousands of square kilometers, is vast. Yet it 

 is rather rare, if not unknown, for a rock which has been deeply buried 

 and approaches the surface by denudation not to have the older openings of 

 all kinds almost entirely filled. 



In proportion as the process of cementation advances it necessarily 

 follows that the openings become smaller, the circulation of the water is 

 retarded, material is transported in smaller quantity, and the process 

 becomes slower. This lessening of speed continues from the first, and 

 when cementation is near completion it must be exceedingly slow. 

 Notwithstanding this the process, as has been explained, has been 

 practically completed for great thicknesses of rocks over extensive areas. 

 The time required for the work must have been great, and yet for 

 completion under favorable conditions it is certain that it does not require 

 geological eras. This is shown by the complete cementation of formations 

 of Eocene age In such instances the process must have been completed 

 and a sufficient time have elapsed for erosion to bring the cemented rock 



