856 A TREATISE ON METAMORPHISM. 



The amount of material required to cement the psephites is usually 

 not nearly so great as that required to transform pure quartzose sands to 

 quartzite. The reason for this is that the material is of such varying sizes 

 that the spaces between the larger blocks are largely filled by finer 

 materials. Also the fine material varies greatly in size. Consequently the 

 amount of original pore space is frequently but a fraction of that of the 

 even-grained sandstones. But in some instances the amount of fine material 

 is not great, and the pore space is correspondingly large. Such deposits 

 are illustrated by the psephites at the mouths of canyons of the West, which 

 are so porous as to permit the absorption and transmission of vast quantities 

 of water. 



During the process of cementation metasomatism may take place. As 

 already noted, there is great variety in the composition of the psephites, 

 and any rock-making mineral maj^ be present. From these there may be 

 produced a great variety of secondary minerals — in fact, any of the minerals 

 mentioned as characteristic of the belt of cementation. (See pp. 621-627). 

 These minerals may be mingled with secondary minerals formed before 

 the material was built into a psephite. It is frequently difficult or 

 impossible to discriminate the alterations which occurred while the minerals 

 were in the primary rock from those which took place after the minerals 

 were in the secondary rock. 



The question now arises as to where the processes of transformation of 

 pebble, sand, and bowlder deposits to conglomerates take place. May a 

 considerable part of the work be done for those rocks which are deposited 

 below water before they emerge from it, or is the work in all cases mainly 

 accomplished when the rocks are below land areas! It is, of course, 

 possible, and indeed probable, that cementation and metasomatism, the 

 important processes in induration, take place to some extent below the sea. 

 However, it is not thought that these processes are there important. One 

 reason for this belief is the certainty that circulation of the waters of the 

 ocean must be comparatively slow through the sediments at the bottom ; and 

 it has been explained on pages 571-572, 866-868, that to dissolve and trans- 

 port a large quantity of mineral material requires a vigorous circulation. 

 A second reason for this belief is that while psephite deposits are still below 

 the sea they have above them no belt in which weathering is taking place, 

 and therefore no belt from which solutions steadily supply material for 



