26 Economic Geology. 



realised its full economical value, since, as far as my infor- 

 mation extends, I am not aware of any instance of perfectly 

 successful boring operations carried on either in transition 

 or primary districts. As before stated, accidental causes 

 may give rise to temporary supplies on such localities, but 

 the hydraulic arrangements essential to the action of an 

 overflowing fountain not being there available, the Artesian 

 system cannot be introduced. Hence then it may be 

 borne in mind as a general principle, that all countries in 

 which secondary or tertiary formations are extensively 

 developed, will furnish those localities in which boring 

 may be employed with the greatest probability of an ade- 

 quate return for the expenditure incurred. 



The locality which perhaps more than any other in the 

 world has tended to give celebrity to Artesian wells, is the 

 tertiary basin of the Thames, the strata of which consist- 

 ing of a series of siliceous, calcareous, and argillaceous 

 deposits, both of mechanical and chemical origin, rest in a 

 basin-shaped hollow of secondary chalk. As this chalk, 

 and also the sandy beds of the basin itself, outcrop at various 

 points, the water falling upon them immediately percolates 

 through them and accumulates in the lower portions of the 

 basin. It here remains under great pressure, since the 

 London clay, a bed of compact adhesive clay several hundred 

 feet in thickness, overlays the whole. The moment therefore 

 this clay is pierced by the auger the water rushes through 

 the aperture with great violence, and the point of supply 

 being considerably elevated above that of issue, an over- 

 flowing well is established. No disposition could be more 

 favourable for boring operations than this, and hence the 

 very general success which has attended them in this and 

 similarly constituted localities has perhaps led to their 

 economical value being over estimated, and over sanguine 

 hopes entertained of their universal applicability. It is to 

 be regretted that the cases of failure have not been recorded 



