128 A TREATISE ON METAMORPHISM. 



ings of autoclastic rocks, etc. In the case of some breccias the pore space 

 is certainly as large as in the mechanical sediments, and such breccias in 

 some places are present in considerable volume. From this maximum 

 amount the pore space of course varies to a fraction of 1 per cent. 



I am therefore wholly unable to give any general averages of the 

 amount of pore space, taking the world as a whole. But Shaler has esti- 

 mated that the amount of igneous and vein material of certain regions of 

 the New England coast is from 3 to 5 per cent of the superficial area. a 

 Since the volumes are as the cubes of the dimensions, if the amount of vein 

 material were the same in other directions this would involve a filled pore 

 space of from 0.52 to 1.12 per cent. 



From the foregoing it is plain that, while it is easy to ascertain the 

 amount of pore space in a given rock, it is very difficult indeed to make any 

 estimate of the average amount of pore space in the zones of katamorphism 

 and anamorphism. It is shown on pages 187-191 that these zones corre- 

 spond, respectively, to the zones of fracture and flowage. It is certain that 

 the pore space in the zone of fracture is far greater than in the zone of flow- 

 age. It is also equally certain that the pore space in the belt of weather- 

 ing is vastly greater than in the belt of cementation. When these various 

 zones and belts are discussed it will be shown that both the unconsolidated 

 materials and the coherent rocks of the belt of weathering are exceedingly 

 open and have a very large pore space. It will further be seen that in 

 passing downward from the belt of weathering to the belt of cementation 

 there is a sudden diminution in the amount of pore space available, the 

 rocks becoming almost at once far less open. Doubtless on the average 

 the amount of pore space in the belt of cementation steadily diminishes 

 from the upper to the lower part; and in the zone of anamorphism the pore 

 space is almost certainly but a fraction of 1 per cent. 



It is to be remembered that below the comparative^ thin belt of 

 weathering, the rocks, with unimportant exceptions, are saturated. Dana 

 estimates the average amount of water contained in the rocks as 2.67 per 

 cent of their weight. 6 Supposing that the specific gravity of the crust is 2.7, 

 this would mean a pore space of 6.89 per cent of the volume of the rocks; 

 or, if the rocks were saturated, about 69 liters of water in every cubic 



a Shaler, N. S., The crenitic hypothesis and mountain building: Science, vol. 11, 1888, p. 281. 

 » Dana, J. D., Manual of geology, American Book Co., 4th ed., 1895, pp. 205, 311. 



