Hawortu. | Physical Properties of the Tertiary. 274 
different. The precipitation rarely exceeds twenty inches annually, 
while frequently it will not exceed ten inches per annum over large 
areas for a period of two or three years. When a rain falls carbon- 
dioxide gas is washed from the atmosphere, the same as elsewhere, 
and the calcium carbonate formed near the surface by previous dis- 
integratior is rendered soluble exactly as elsewhere. But the pre- 
cipitation is so light and the sand beds and clays are so dry that the 
water, working its way downward, is largely absorbed by the dry 
earth before it connects with the general underground water below. 
Such a desiccation process would leave the calcium carbonate 
wherever it chanced to be; that is, it would leave it scattered 
throughout the sands and clays. ‘The gravel beds and sand beds, 
on account of their greater porosity, would permit the circulation 
of water more freely than the clays. Asa result the ground water 
near the surface in its downward migrations would follow such beds 
more frequently and in greater volume than the clay beds. Whea 
a horizon was reached composed principally of coarse sand and 
gravel a lateral movement would result from the clay beneath acting 
as an obstruction to downward movement. The extent of the 
lateral movement would depend upon the amount of water and upon 
the distance necessary to be passed over before a downward outlet 
could be found. It has already been seen that the sand and gravel 
beds form large parts of the Tertiary areas. The downward move. 
ment of the water from the rains would be very irregular. Where 
the channels were most open, that is in the gravels and clays, the 
movement might extend laterally for long distances. If the gravel 
beds are irregular in their stratigraphic relations so that almost 
every one comes near the surface at one place or another, it is not 
unreasonable to suppose that every gravel bed has had an oppor- 
tunity to serve as a passageway for waters coming from the surface 
heavily charged with acid calcium carbonate. The ultimate drying 
up of such waters would deposit the carbonate in the beds of sand 
and gravel, the same as elsewhere. If a larger proportion of water 
traveled through them we would expect a correspondingly larger 
deposition of the carbonate. 
It is a noteworthy fact that the sand beds and gravel beds which 
earry such large quantities of underground water rarely if ever 
