“ We found that 'the cut-off point where 'gross income from mining' stopped has been the same' ever since the first depletion statute, namely, 'where the ordinary miner shipped the product of his mine.' Id., at 87, 80 S.Ct., at 1587. It therefore appears from this record that the 'product' with which the Code deals here is the taxpayer's product at the point when 'mining' terminated, i.e., when it reached the crushed limestone stage. [2] This results in limiting the taxpayer's basis for depletion to its constructive income from crushed limestone, rather than from finished cement. ”