Economic Geology. 25 



above the plain, then the sandstone under the clay would 

 certainly be charged with water, and a bore carried through 

 the latter would have every probability of success. The 

 water would rise with a force proportional to the perpendi- 

 cular height of the source of supply above the point of is- 

 sue, and it may be remarked, that if there are good grounds 

 for believing the porous stratum to be continuous for a con- 

 siderable distance, or to communicate directly with other 

 porous strata, it is not necessary to sink the well in the im- 

 mediate vicinity of the hill, and it would be advantageous, 

 especially in countries subject to periodical droughts, to 

 have as great an interval as might be compatible with the 

 success of the project between the well and its source of 

 supply, since there would then be the less risk of exhaustion. 

 It is quite possible that when the auger brings up sand 

 the water may not immediately rise, since it may have per- 

 colated only to the lower portion of the porous bed, but the 

 disposition is favourable, and perseverance will ultimately 

 ensure success by driving the bore through the sandstone 

 till it reaches the water line. There is a farther possibility 

 that the sandstone instead of reposing on an impervious bed, 

 may be underlaid by a pervious one, and to this the water 

 may have penetrated. Still however the probabilities are 

 in favour of success, since in alternating beds an impervious 

 one may reasonably be expected to occur after two or more 

 pervious strata have been met with. Throughout nearly the 

 entire series of secondary rocks, especially in the oolitic and 

 cretaceous groups, such alternations occur, and boring has 

 been successfully employed in many localities where these 

 formations alone prevail. A similar succession of loose 

 arenaceous or calcareous with compact impervious argilla- 

 ceous deposits characterises tertiary formations, and these 

 being disposed chiefly in basin-shaped hollows, have been 

 found peculiarly adapted for Artesian wells. It is only 

 indeed in secondary and tertiary formations that boring has 



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