RTE LE ROPRERISES OF BOULLDING STONES, ETC. L7i 
The size, shape, manner of contact, and arrangement of the 
grains control the size of the pores and the percentage of pore 
space.’ The porosity of a rock is an important factor, influen- 
cing the danger from alternate freezing and thawing of included 
water. 
The pores, or spaces between the grains, which are connected 
in such a manner as to allow water to‘ flow from one part to 
another have been divided for convenience into three classes. 
The first class consists of small interspaces that exist between 
the grains of a rock, known as pore spaces; the second class 
consists of those openings which form along bedding, jointing, 
and fissile planes, known as sheet openings ; the third class are 
those openings caused by the removal of several or many of the 
individual grains, commonly known as cavities, caves, or cav- 
erns. All of these openings frequently occur in the same 
rock. 
Pores are ordinarily conceived of as being connected so as 
to form irregular-shaped tubes. Naturally they differ very 
greatly in size, depending upon the fineness and shape of the 
original particles composing the rock and the extent to which 
the interstices have been filled with secondary mineral matter. 
In the same rock all the pores are never of the same size, 
although they may have a general correspondence in size. The 
pores spaces are classified according to size into capillary and sub- 
capillary. The capillary pores are the larger and the water which 
they hold is known as the water of saturation. Openings 
included in this class are over .00002 centimeter in diameter? 
If a rock containing capillary pores is allowed to drain off natur- 
ally, a portion of the water will escape, but another portion will 
*It has been pointed out in another place that pore space in sedimentary rocks 
depends largely upon the size and shape of the grains and the amount of cement. In 
general this is true, but the cement itself becomes an individual grain, when once 
deposited in the interspace of a rock, and the shape and size of the cement grains 
should be considered. All particles of which a rock is composed should receive con- 
sideration as constituent grains of the rock. 
?Metamorphism of Rocks and Rock Flowage, C. R. VAN Hiss, Bulletin of the 
Geological Society of America, Vol. IN, p. 272. 
