ARTESIAN WATERS IN THE ARID REGION 603 



the possibility of securing water at any given point must be 

 determined by the local formations. All rocks imbibe moisture 

 in proportions varying with their physical structure, a fact which 

 can be demonstrated experimentally by saturating familar types 

 of rocks. Glass is similar in water capacity to large areas of 

 volcanic and other igneous rocks, and will absorb no perceptible 

 amount of moisture ; marble will drink in only a slight quantity ; 

 while chalk, sand, and brick will absorb nearly their own weight 

 of water. The manner in which rocks absorb water is simple. 



Fig. 3. — Mesa Structure of Llano Estacado. 



In most rocks, however compact to the eye, there exist interstices, 

 cavities, and other spaces in which water may enter and be stored. 

 This is especially true of all sedimentary rocks, which comprise 

 ninety-nine per cent of the earth's crust. A fine sandstone whose 

 grains and intervening spaces are indistinguishable to the eye, 

 when placed under the microscope resembles a mass of cobble- 

 stones in which the spaces occupy as much of the aggregate area 

 as the solid particles. Into a gallon measure of dry pebbles vary- 

 ing in size may be poured half a gallon of water. The consoli- 

 dated rocks which compose most of the mountain masses are 

 more compact and less adapted for the storage and passage of 

 water than the sedimentary rocks. Nearly all the minerals which 

 compose them are impervious, as is readily seen in a large crystal 

 of quartz, feldspar, or mica. The rocks of valleys and plains 

 usually consist of detrital material less hardened by mountain- 

 folding, and hence more pervious. 



Rocks which have imbibed all the moisture they can contain 

 are in a condition of saturation, and all water in excess of this 



Fig. 4. — Artesian Functions of Impervious and Pervious Strata. 



amount will pass off by gravity or evaporation. The excess 

 above the water of saturation is available as the source of springs, 

 but the supply of wells is from the water of saturation. 



Each kind of rock has an individual capacity for the trans- 

 mission of the water which it has imbibed, and this is entirely 



