impervious clay. Any part of the porous sands not filled with petroleum or with the gas arising therefrom, was filled with water, most of which contained salts such as sodium sulphate, sodium chloride, magnesium sulphate, etc.; the oil being lighter than water would tend to float on the surface of the latter within the sands, and would always take up a higher position than the water unless prevented.
As the folding movement proceeded, these porous beds
32. The Yenangyat Anticline, showing the arch.
with their protecting caps of clay became warped and folded into arches and troughs, the oil finding its way into the crests of the arches and the water occupying the troughs. In some cases the arches were folded so severely as to be fractured, most of the imprisoned oil thereby escaping. In places like Yenangyaung in the Magwe district, on the other hand, the arch is a gentle and undisturbed one, and vast quantities of petroleum have collected in the many porous sandstones beneath it. On this oil is exerted hydro-
