586 A TREATISE ON METAMORPHISM. 



water, in order that the new water shall have room to enter the sandstone 

 without raising the water level, should be three-fourths of 1 kilometer per 

 annum. If the dip of the artesian-bearing strata were steeper the lateral 

 movement would be much less. The above case, however, is approxi- 

 mately that of the Potsdam sandstone of Wisconsin. Since the average 

 lateral movement of the water can not be supposed to be more than 

 three-fourths of 1 kilometer per annum, the water which enters the artesian 

 circulation at a distance of 150 kilometers from Chicago should, on the 

 average, issue from the wells at Chicago 200 years after it enters the sea 

 of ground "water in Wisconsin. 



In so far as the openings in the porous stratum are not of uniform size, 

 the movement will be more rapid through the larger openings, and so far 

 as this takes place the water rising in the artesian wells would remain 

 underground for a less time than calculated; but, on the other hand, the 

 remainder of the water which did not follow the larger openings would 

 travel at a slower rate than calculated, and thus remain underground a 

 longer time; and thus, so far as difference in porosity is concerned, the 

 average would be as calculated. 



The average length of time during which the water remains in the 

 ground would probably be longer, perhaps much longer, than calculated, 

 for all the premises are made so as to give the minimum time. For 

 instance, the calculation is made upon the supposition that the porous 

 stratum is homogenous and that there is no leakage through the overlying 

 impervious structure, whereas it is certain that there is leakage. In so far 

 as there is leakage the lateral movement would be slower and the calculated 

 length of time should be increased. 



Also the annual rainfall which enters the sea of ground water in the 

 belt of cementation is not so great as 25 centimeters, for a considerable 

 portion of the water which goes underground is evaporated from the belt 

 of weathering. In so far as the amount is overestimated the' length of time 

 calculated — two hundred years — should be increased. 



The above result as to length of time which water remains underground 

 in an artesian circulation is not so surprising when we compare it with the 

 exceedingly slow flow already mentioned for the underground water in the 

 sands of the Arkansas River and of the alluvial cones of southern Cali- 

 fornia, in which the descent of the level of ground water is many times 



