GENERAL CONSIDERATION. J 



by marine strata, and they further differ from them by having a 

 well-marked original slope of deposition. Their composition varies 

 a great deal from place to place, and the irregularity thus produced 

 may be favourable to the production of artesian conditions. Vol- 

 canic formations accumulated on land are comparable to these 

 alluvial formations in their consisting also of strata of varying per- 

 meability deposited in an inclined position. As to the floor of 

 igneous and metamorphic rocks which carries the strata of either 

 Rocks unsuited to category, it is practically impermeable. Occa- 



artesian conditions. sionally it has been known to yield a somewhat 

 abundant supply of water through fissures, but such occurrences are 

 so rare and so undependable that they cannot be taken into account 

 in any practical scheme. 



In regions of great disturbance, sedimentary strata may be so 

 much compressed that they become impermeable to the same extent 

 as these igneous and metamorphic rocks. But apart from such 

 cases, all the sediments whose relative age can be determined 

 by means of fossils, from the oldest cambrian to the newest 

 tertiary, have yielded artesian flows under favourable conditions. 

 On account of the great regularity of the strata composing them, 

 artesian reservoirs formed by the tilting of these ancient marine 

 sediments, "basins of disturbance" in fact, lend themselves to 

 systematic investigation in such a manner that in many cases it has 

 been possible to foretell with a fair degree of certainty the prospects 

 of a water-supply at any particular point. But with undisturbed 

 alluvial, deposits that are still in the same position in which they 

 were originally laid down, we have no means of studying the con- 

 cealed strata except by artificial excavations, and it is always difficult 

 to give a decisive opinion. Wells sunk in such a formation are 

 always more or less of an experimental nature. 



In strata belonging to this category, it is sometimes difficult to 

 decide whether a well is really or not of artesian nature. The re- 

 striction of the name to flowing wells is not a satisfactory one since 



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