Page:Popular Science Monthly Volume 72.djvu/513

Rh and texture of the formation, also because of its subjacency to horizons that carry water freely.

In the Logan formation, I have mapped thirty houses with springs. There is doubt concerning a few of these, an indefiniteness occasioned by the absence of contacts. The Logan sediments suffered erosion contemporaneously with Pottsville sedimentation; furthermore, the Logan, in comparison with its contact formations, the Black Hand and the Sharon, weathers easily, producing gentle slopes. These two conditions make it doubtful about the exact horizon of a spring near either the top or the base of the Logan.

Slightly less than seventeen per cent, of the houses with springs are found in the Sharon. The areal extent of all the exposed formations diminishes vertically, hence the number and the volume of the springs decrease; the value of the land for farming also decreases with altitude. A further fact concerning the springs of the Sharon is their content of iron, making them less desirable than springs in either of the lower formations.