DECOMPOSITION OF GRANITOID ROCKS.
General Agents and Processes.—§ 12. All rocks and minerals are porous, that is, the particles of which they are composed cannot lie against each other in such a way as to occupy all the space, and hence openings or pores are left for the passage of air or water. This is as true of a crystal of quartz or a diamond as it is of the coarsest sandstone. If a crystal or fragment of any stone be dried for several hours at a temperature above 212 deg. F., carefully weighed by a delicate balance, then submerged in water and either placed in a receiver from which the air has been exhausted or boiled for several hours, then taken from the water, the surface carefully dried, and the specimen weighed again, the latter weight will be greater than the former by the weight of water which it has absorbed. This excess of weight used in connection with the specific gravity and the dry weight of the specimen, will give the pore space.
(Wet weight—dry weight) x specific gravity — = percentage dry weight of pore space.
Van Hise in his "Treatise on Metamorphism"* states that the pore space varies from a small fraction of 1% to 50% or more, and gives a very pretty experiment to prove the presence of pores in an apparently impervious material. In agate or chalcedony the pores are so small that the most powerful microscope fails to detect them, yet if thoroughly dried specimens be boiled in colored solutions the liquid will make its way into them and change their color.
The average pore space in building stories is variously estimated but as data are insufficient but little reliance can be placed upon any of them. Perhaps as fair an estimate as has been proposed for all rocks is 13% which would enable them to absorb about one gallon of water for each cubic foot. This may or may not be near the truth, but it serves to illustrate the fact that apparently solid rocks when saturated carry large volumes of water. It should also be remembered that this water is not confined to the inter-crystalline spaces, but permeates the inter molecular spaces as well, even when they are so small that they cannot be seen by the aid of the microscope.
§ 13. Under the influence of gravity, changes in molecular attraction, temperature, and mechanical action, this water is continually kept in motion, but the velocity of movement varies widely. In coarse-grained rocks with large pore spaces, lying above the level of ground water, especially if cut by fissures or other openings produced by mechanical or chemical action, the flow would be relatively rapid, but in proportion as the rocks become finer-grained, the grains more angular, the structure more compact and the opportunities for drainage less, the rate of move ment is reduced until it becomes so low as to be unmeasurable by the means at our disposal. Not only this, but the flow varies widely within
the rock-mass itself, being relatively rapid in the inter-crystalline or inter-granular spaces and very slow in the pores between the molecules.
When the rock lies near the surface, well above the level of ground water, and is more or less cut up by joints, bedding planes and planes of cleavage or fracture, gravity becomes the dominant cause of motion and the water flows downward toward the base of the mountain or the immediate valley of some stream, but when it lies below the level of ground water, other forces often become strong enough to overcome grav ity entirely and the water flows in the direction of the least resistance, whether that be upward, sidewise or downward. Under these condi tions water often rises from great depths bringing increased temperature and a load of dissolved material which greatly increases its working power.
The water which circulates near the surface is almost entirely de rived from rain. As water falls through the air it dissolves small quantities of carbonic and other acids and so reaches the surface in the form of a weak acid solution. The rain-water which falls upon the sur face may be divided into three portions, of which the first, much the smallest, is immediately converted into vapor and rises; the second, the run-off, does not enter the ground but slides off into nearby streams, while the remainder sinks into the ground and fills the pores referred to above. This ground-water represents a varying percentage. of the rainfall, depending on the physical condition of the surface layers. It may represent practically all the water that falls or only a very small fraction, but, except in the driest regions, it is always sufficient to keep the surface layers moist and the chemical forces active. As rain-water enters the rocks it carries its dissolved acids with it and so brings them into contact not only with each rock particle, but also with each molecule of which these particles are composed, and as this contact usually occurs under conditions which make carbonic acid stronger than silicic acid, a reaction takes place and the former replaces the latter. The table of rock-forming minerals given above shows that they are usually com plex salts in which several bases are united with silicic acid. Some of these bases are more strongly held by the acid than others, and it hap pens that those held with least force by silicic acid are most strongly attracted by carbonic acid. Potash and soda seem to be attacked first, then line and magnesia, then iron, and lastly alumina.