GEOLOGICAL WORK OF GROUND WATER. 157 



already been shown. Since the horizontal journey is, on the average, long 

 as compared with the vertical journey, the element of time is of much 

 greater importance in the horizontal component of the journey than in the 

 vertical component. The capacity for geological work is increased by 

 pressure and by temperature. These forces, under ordinary conditions, are 

 a function of depth, and these factors in the work mainly concern the ver- 

 tical component of movement. During the downward journey the pressure 

 and temperature steadily increase, and the amount of material in solution 

 increases. During the upward journey the pressure and temperature 

 diminish and the tendency for material to pass from solution or to be 

 precipitated increases, and the amount held in solution diminishes. 



Pressure and temperature are ever working- together according to 

 definite laws. Both increase in efficiency with depth, and they greatly 

 promote the activity of deep ground waters. However, of all the vary- 

 ing factors, varying temperature is the one which is of incomparably 

 the greatest importance. High temperature ordinarily results from depth 

 of penetration; but it has been pointed out that it may result from various 

 other causes, of which chemical action, mechanical action, and the pres- 

 ence of intrusive igneous rocks are the more important. The capacity 

 which water has for taking and holding in solution various relatively 

 insoluble compounds, and the velocity of chemical reactions, increase 

 enormously with increase of temperature. Not only is high temperature 

 favorable to geological work, because of the chemical activity of the water, 

 but, as already pointed out, high temperature greatly decreases its viscosity, 

 and this, as already explained, is favorable to depth of penetration and 

 flow through minute openings. Since the temperature changes of ground 

 water are commonly dependent upon depth, the vertical component of the 

 movement of underground water is ordinarily far more important than the 

 longer horizontal component. 



The underground journey of water may occupy hundreds of years. 

 (See pp. 585-586.) The surface of contact in very small openings is 

 very great. Under these conditions of slow movement and small openings 

 there is sufficient time nearly to establish complete equilibrium between 

 the solutions and the solids with which they are in contact; but it has been 

 seen (pp. 34-35) that rarely or never is the adjustment of a rock to its 



