UNDERGROUND WATER 147 
noticed that they all illustrate the passage of water 
through rocks;.a passage which, in every case, some- 
times by changing and sometimes by dissolving some of 
the minerals, leaves the rock slightly modified. The 
springs, and the water which is slowly seeping through 
the rocks, give to the stream 
a load of dissolved mineral 
to be carried from the land 
to the seas. 
Artesian Wells. — While 
water is percolating 
through all the rocks, its 
passage through some is 
: Fig. 73. 
easier than through others. Diagram to illustrate formation of a 
A loose stratum, composed Sing os sie where porous 
of sand grains, 1s more per- 
meable than a dense one made of fine grains of clay. 
In many parts of the world this difference gives rise 
to peculiar conditions, which are of value to man in 
furnishing a supply of pure water from deep within 
the earth. 
Where first found, in the Province of Artois in 
France (Fig. 74), the conditions were as follows: a 
sandy layer of rock had been so bent that the two 
ends extended to the air, while the lower portion was 
deep in the earth. Above and below it were layers 
of impervious rock. Where the sandy stratum was 
