22 Economic Geology. 



influence of their as yet mysterious attractive affinities be- 

 come aggregated in its higher regions in the form of clouds. 

 These are distributed through the instrumentality of the 

 winds over the surface of the earth, and according to 

 circumstances descend either as refreshing showers, or at- 

 tracted by mountain ranges serve to feed the numerous 

 streams which flow from these, or absorbed by the earth 

 itself as they pass through its porous strata, and form those 

 internal reservoirs whence natural and artificial fountains 

 derive their supplies. It is with the latter case alone we 

 have at present to do, and the geological relations by which 

 it is accompanied will now be examined. 



Since the diameter of an Artesian well is seldom more 

 than six or eight inches, it is evident that to make it econo- 

 mically valuable, the water must not only rise in it, but must 

 also overflow with some force at the surface. Such overflow- 

 ing is indeed the characteristic of an Artesian in contra- 

 distinction to a common well, the supply in the latter maintain- 

 ing a stationary level, and its diameter being necessarily con- 

 siderable. In order that the water in an Artesian well may 

 thus rise above the surface, it is essential that hydrostatic 

 pressure should be exerted upon it, and to this end a 

 certain disposition of strata is necessary. The important 

 connection between the geological structure of any particu- 

 lar locality and its adaptation for boring operations is thus 

 apparent, and in passing we may notice the incorrectness of 

 the opinions not unfrequently advanced, that all localities 

 are alike, and that since Artesian wells have succeeded in a 

 great many places, they are therefore applicable in all. A 

 very limited acquaintance with the peculiarities of the 

 earth's structure, and of the arrangements of the beds com- 

 posing its crust, would suffice to shew how untenable such 

 views are. 



Districts in which the class of rocks usually denominated 

 primary, prevail, to the exclusion of those of later formations, 



