All mines, except mines worked or operated primarily for coal, asphaltum, rock, limestone, dolomite, or other stone products, sand, gravel, clay, or earths, shall, for the purpose of valuation for assessment, be divided into two classes: Producing and nonproducing.
Source: L. 65: R&RE, p. 1102, � 1. C.R.S. 1963: � 137-6-3.