18, 3 Smith: Tropical Geology and Engineering 233 
Some rocks, like the dense volcanic flows and quartzites, con- 
tain practically no water. Other rocks, like the limestone for- 
mations, have subterranean rivers passing through them, but 
the sandstones and chalks are the most important in this respect. 
A true sandstone consists of grains of quartz compacted together. 
The more nearly spherical these grains are, the more continuous 
are the openings, ignoring for the moment the cementing mate- 
rial which in some cases may completely close the voids. In 
such an arrangement of spherical grains there is always one 
continuous passage through the mass. Along these passages 
the water travels. Some sandstones have as high as 35 per cent 
of pore space. 
The loose volcanic tuff formations of the Philippines are also 
great water-carrying formations. This water is generally cir- 
culating slowly in the small openings, but when the water reaches 
a fissure, naturally the circulation is rapid. 
As the deep-seated igneous rocks are usually dense and have 
but little pore space, and as the fractures in these rocks have 
no regularity, it can be seen at once that they are poor forma- 
tions in which to seek water. Water may be traveling along a 
fissure in such formations, and the well digger may perchance 
strike this fissure, but usually the chances are slim. There is 
no excuse for locating a well in such a formation, unless this 
is the only kind of rock in the region. A limestone formation 
is also poor, as a rule; for, while it is cavernous and contains 
underground streams, it is a piece of pure luck when one of these 
is struck. Pratt,® a former colleague in the Philippine geolo- 
gical work, found that where coral limestone was interbedded 
with clay layers a fairly good supply of water was available. 
Very often, however, shallow wells in limestone areas located 
near the sea would yield salt water or be dry in about half the 
cases. The best conditions are found in the sandstone, volcanic 
tuffs, and beds of unconsolidated ash. 
Having found the formation, we must next consider some 
features in the structure of the region. The ideal condition 
is where the sediments form a syncline (basin), with the water- 
bearing stratum outcropping at a higher point than the site of 
the well and where there is an impervious layer of clay or shale 
just above the water-bearing stratum. The effect of a fault 
‘on the water circulation would be to divert the flow to springs 
at the outcrop, but the effect of an igneous intrusion would be 
* Pratt, W. E., Philip. Journ. Sci. 10 § A (1915) 236. 
