566 A TREATISE ON METAMORPHISM. 



surface contour by the depth of the level of ground water. It has also 

 been seen (pp. 411-413) that the surface of ground water rises and falls 

 from a few centimeters to a number of meters, and that therefore the apper 

 boundary of the belt of cementation is somewhat variable. 



The lower limit of the belt of cementation is the lowest horizon at 

 which there may exist abundant openings in the rocks of supercapillary and 

 capillary size. It has already been pointed out (pp. 190-191) that to this 

 limit the rocks are self-supporting, but below it the rocks are not sufficiently 

 strong to support themselves, so that if openings were supposed to be 

 produced they would be closed by flow." It has been calculated that for 

 the strongest rocks the bottom of the belt of cementation may be as deep 

 as 10,000 or possibly 12,000 meters, although for most rocks under ordinary 

 conditions the bottom of the belt of cementation is believed not to be nearly 

 so low. With the same kind of rock the bottom of the belt of cementation 

 is at different depths under different conditions. For instance, when earth 

 movements are rapid the belt might extend considerably deeper than when 

 they are slow. The belt of cementation, extending as it does from the level 

 of ground water to the bottom of the zone of fracture, is much broader 

 than the belt of weathering. 



CONDITION OF WATER IN BELT OF CEMENTATION. 



The question whether hydrostatic pressure increases sufficiently fast 

 with depth to prevent the water from passing into the form of gas needs to 

 be answered. If the average temperature at -the surface be assumed to be 

 0° C. — and in the arctic regions the average temperature is probably lower 

 rather than higher than this — and if the increment of increase of tempera- 

 ture be taken as 1° C. for every 30 meters, the critical temperature of water, 

 365° C, is at a depth of 10,950 meters. If the average temperature at the 

 surface were supposed to be 25° C, about the maximum for the tropical 

 regions, in order to reach a temperature of 365°, the critical temperature of 

 water, a depth of 10,200 meters would be required. 



At any given place the water is subject to the pressure of the super- 

 incumbent column of water. Supposing the temperature of the water were 

 100° C, or just at the boiling point, at the surface of the earth (the most 



"Van Hise, C. R., Principles of North American pre-Cambrian geology: Sixteenth Ann. 

 Eept. XJ. S. Geol. Survey, pt. 1, 1896, pp. 591-594. 



