CLASSIFICATION OF SPRINGS 



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explanations improbable. These springs, therefore, may be taken 

 as typical of the class of fracture artesian springs. 



D. SPRINGS IN IMPERVIOUS ROCK 



The term porous and impervious are of course relative, but 

 impervious rocks may be considered to have pore spaces of capil- 

 lary or smaller size, which usually form a relatively small percentage 

 of the rock. Some clay, however, has a high porosity but because 

 of the absorption of water and swelling of individual grains is 

 highly impervious. Through such pore spaces effective flow 

 under hydrostatic head is impossible. Movement takes place 



A. Solution tubular spring 



B. Minor tubular spring 



Fig. 21. — Diagram illustrating two classes of tubular springs: a, solution tubular 

 springs; b, minor tubular springs. 



through openings which may be grouped into two types: (i) those 

 which are more or less tubular in form, and (2) those which are 

 sheetlike in form and along which the water moves as a thin film. 

 I. Tubular Springs. — Springs of the first group may be called 

 tubular springs. This group can in turn be divided into three 

 classes, according to the origin of the tubular openings. Solution 

 tubular or cavern springs are due to channels and openings formed 

 through solution of the rock by circulating ground water (Fig. 21 a). 

 Solution commonly begins along joints or other previously existing 

 openings, and the channels may be enlarged to a very great size. 

 In these tubes the water then flows freely and with relative rapidity. 



