Petroleum. Promotion of mining on public domain, hearings, 74th Congress, 1st session, on S. 1772, to amend act to promote mining of coal, phosphate, oil, oil shale, gas, and sodium on public domain (41 Stat. 437, U. S. C., title 30, sec. 185, 221, 223, 226), as amended [relating to oil and gas], Apr. 24-30, 1935.