
346 , DYNAMICAL GEOLOGY. [Book iia 
Since rocks vary greatly in porosity, some contain far more water 
than others. It often happens that, percolating along some porous 
_ bed, subterranean water finds its way downward until it passes under 
- some more impervious rock. Hindered in its progress, it accumulates 
in the porous bed, from which it may be able to find its way up to 
the surface again only by a tedious circuitous passage. If, however, 
a bore-hole be sunk through the upper impervious bed down to the 
water-charged stratum below, the water will avail itself of this 


Fig. 98.—DIAGRAM ILLUSTRATIVE OF THE THEORY OF ARTESIAN WELLS. 
a, b, Lower water-bearing rocks, covered by an impervious series (c), through which, 
at 1 and elsewhere boring’s are made to the water level beneath. 
artificial channel of escape, and will rise in the hole, or even gush 
out as a jet d’eaw above ground. Wells of this kind are now largely 
employed. They bear the name of Artescan, from the old province 
of Artois in Franee, where they have long been in use.! 
That the water really circulates underground, and passes not 
merely through the pores of the rocks but in crevices and tunnels, 
which it has no doubt toa large extent opened for itself along natural 
joints and fissures, is proved by the occasional rise of leaves, twigs, 
and even live fish, in the shaft of an Artesian well. Such testimony 
is particularly striking when found in districts without surface waters, 
and even perhaps with little or no rain. It has been met with, for 
instance, in sinking wells in some of the sandy deserts on the southern 
borders of Algeria.? In these and similar cases it is clear that the 
water may, and sometimes does, travel for many leagues underground 
away from the district where it fell as rain or snow, or where it leaked 
from the bed of a river or lake. 
The temperature of springs affords a convenient, but not 
always quite reliable indication of the relative depth from which they 
have risen. Some springs are just one degree or less above the 
temperature of ice (C. 0°, Fahr. 32°). Others in volcanic districts 
issue with the temperature of boiling water (C. 100°, Fahr. 212°), 
Between these two extremes every degree may be registered. Very 
cold springs may be regarded as probably deriving their supply 
from cold or snow-covered mountains. Certain exceptional cases, 
however, occur where ice forms in caverns (glacieres) even in 
warm and comparatively low districts. Water issuing from these 
ice-caves is of course cold.* On the other hand, springs whose 
temperature is higher than the mean temperature of the places 
at which they emerge must have been warmed by the internal 
' Sce Prestwich, Q. J. Geol. Soc. xxviii. p. lvii. and the references there given. 
2 Desor, Bull. Soc. Sci. Nat. Neufchdatel, 1864. 
* The most remarkable example of a glacitre yet observed is that of Dobschau, in 
Hungary, of which an account, with a series of interesting drawings, was published in 
1874 by Dr. J. A. Krenner, keeper of the national museum in Buda-Pesth. 
