246 GHOLOGICAL SURVEY 
The rain water that falls upon the earth’s surface, either passes of directly off the surface to a lower 
level, or sinks into the porous beds till it meets with one that is impervious; this it follows to the 
outcrop, where it vents itself in one or more springs. ‘The line of outcrop of such impervious strata 
may often be traced along hill sides by means of these springs. Where the impervious strata occur 
near the surface, and the country has but little inclination, swamps are often the result, and where they 
are found far below it, the opposite effect is produced. Such alternations of porous and impermiable 
beds are common in every couniry,* as is shown by the frequency of ordinary wells, which are de- 
pendent upon this structure, for their supply. 
Where the rocks are highly inclined, so that their upturned edges form the surface of the country, 
it rarely suffers from excess of water, which finds a ready passage between the strata, particularly 
if they are slaty; but regions so situated are apt to suffer from the opposite extreme, as was painfully 
illustrated in the slaty portions of Spartanburg and the adjoining districts, during the late exces- 
sively dry season; they suffered even more than the sandy parts of the State. The reverse of this 
takes place where the rocks are horizontal or impervious. ‘The immense tablets of gneiss that cap 
the mountains of the State, where their surfaces are level give rise to miniature swamps, as may be 
seen on the top of Table rock, and Whiteside. 
When a porous bed of considerable extent, one for instance that is made up of sand and pebbles, 
occurs between too impervious strata, an immeuse reservoir is formed, which is filled by the rains, 
if the strata are inclined, at the extremity where the porous bed comes to the surface. If the super- 
incumbent impervious bed be perforated so as to tap the reservoir, the water will rise to the level, 
or nearly so, of the highest point of the reservoir. If the point where the overlying bed was perfor- 
ated be lower than the upper edge of the reservoir, the water, of course, will flow out at the surface, 
or even rise to a considerable height above it, dependent upon the difference of level just stated.t 
Such fountains are known as Artesian wells. 
It will appear from the geological structure of the Charleston basin, that it presents all the condi- 
tions found in those places where borings for these wells have been most successful. In a report 
based upon this fact, which I had the honor of presenting to the Mayor and City Council of Charles- 
ton, recommending an attempt to procure water for the city, by boring, I stated 600 feet as the pro- 
bable depth that would be required to reach the water-bearing bed. Since this was written, a well 
has been commenced in the city, and at Fort Sumter, under the direction of Capt. Bowman. At the 
Fort, the green sand which underlies the Ashley beds appeared to have been reached at somewhere 
near 300 feet, which was the depth of the well at my last visit to the place. 
This corresponds with the position I had assigned that bed, but the proportion of green sand was 
not as great as that in which it is found on the Santee; still it was highly interesting, as the only 
green sand found, occurs at the junction of the Ashley and Cooper and Santee beds, and is seen 
again thinning out in Orangeburg. In a new district, the depth assigned for Artesian wells must 
be taken with considerable latitude. 
Previous to the celebrated well at Grenelle, 1000 feet was the greatest depth reached by any suc- 
cessful attempt at boring for water. The geologists at whose instance this well was commenced, by 
the French government, had calculated 1200 to 1500 feet as the probable depth at which the water 
* And hence the great success of those who seek for veins of water, by means of the divining rod. 
+ From ‘ Artois,’ a French province, where they have been in use for 600 yearse 
