“ An economic interest is possessed in every case in which the taxpayer has acquired, by investment, any interest in mineral in place or standing timber and secures, by any form of legal relationship, income derived from the severance and sale of the mineral or timber, to which he must look for a return of his capital. But a person who has no capital investment in the mineral deposit or standing timber does not possess an economic interest merely because, through a contractual relation to the owner, he possesses a mere economic advantage derived from production. ”

“ While there are income incidents to the utilization of natural resources, there is also an obvious exhaustion of the capital used to produce the income. In theory the aggregate sum allowed for depletion would equal the value of the natural resource at the time of its acquisition by the taxpayer, so that at the exhaustion of the resource the taxpayer would have recovered through depletion exactly his investment. ”

“ If the additional payment in these leases had been a portion of the gross receipts from the sale of the oil extracted by the lessees instead of a portion of the net profits, there would have been no doubt as to the economic interest of the lessors in such oil. This would be an oil royalty. The lessors' economic interest in the oil is no less when their right is to share a net profit. ”