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C. If a rock stratum is not of even incremental thickness <br /> e.g. , 8 feet of sandstone, the stratum should be sampled <br /> from 0-4 ' and 4-8 ' . <br /> d. When an obvious change in properties, such as penetra- <br /> tion of the weathered zone below the land surface, oc- <br /> curs at a point within a rock type, the two zones are <br /> recorded and sampled as different rock members. <br /> 3. After proper subdivision, each entire sample unit is pul- <br /> verized to pass a 10 mesh (2 . Omm) sieve. <br /> 4. The sample is then riffled to obtain a representative <br /> sample of approximately 500 gms. and then dried at 50- <br /> 60°C. overnight. <br /> 5. The sample is then pulverized to pass a 60 mesh (0. 25mm) <br /> sieve and riffled to approximately 100 gms. and retained <br /> in 4 oz. sample bottles . <br /> 6 . The remaining sample is retained in metal containers and <br /> stored in a dry and convenient area for future referral . <br /> 6-2-3 <br />