Sediments tentatively assigned to the Marinoan Series (late pre Cambrian age) unconformably underlie Mesozoic and Cainozoic sediments in the area investigated. Opal is known to occur at three horizons and may occur in a fourth within the Mesozoic sequence, the most important and persistent opal bearing horizon is that having an average R.L. of 1000 feet. Structure form lines indicate a bread basin like structure at the opal field. The discovery of opal 8 miles south east of the existing field and the finding of opal fleaters within 3½ miles of White Dam indicates approximately 80 square miles of potential opal bearing country.