Page 37 - Petrology of Sedimentary Rocks

areal distribution of the different grain size classes. If the area has gravel, gravel
porportions can be contoured in a similar fashion.

Size Analysis by Sieving

It is assumed that the sand has already been disaggregated and that clay or mud,
if present in considerable amounts, have been removed. These methods have been
described in previous sections.

I. Using a sample splitter, obtain about 30 to 70 grams of sample. If there are
numerous screens to be used in the analysis, use the larger weight; if only 4 to 6
screens, use the smaller weight (this is to prevent clogging the screens with too
much sand, a frequent cause of poor results). If there is any gravel (material
coarser than 2 mm), see note at end. Spread the sand evenly along the hopper and
stir it well to mix it thoroughly (do not shake as this will make the finer grains
travel to one end).

2. Weigh the split sample to 0.01 gram.
3. Select the screens to be used. For accurate work, use the yb 4 set; for rough work,
use the h $ set. If you are doing any research, it is senseless to use the I$ interval
as this is too broad an interval to get useful data. The student sets are I@, but
these are just for practice. Clean the screens thoroughly using the procedure
below. This is especially important if you are going to make mineral studies on
the samples after screening.

4. Nest the screens in order, coarsest at the top, pan on the bottom. If the stack is
too big to fit into the Ro-Tap, it will have to be sieved in several stacks, starting
with the coarsest sizes. Pour the sample in to the top sieve and shake gently by
hand. Remove all the screens that are too coarse to catch any grains. Place
cover on the stack.

5. Place the screens in the Ro-Tap, FASTEN VERY TIGHTLY, and sieve for I5
minutes. For all scientific work, the Ro-Tap machine must be used, 8 inch
diameter screens must be used, and the sieving time should be constant for all
samples (I5 minutes is the accepted time). For the small 3” student sets, sieving
rnust be done by hand, using a rotary motion with a bump.

If the analysis must be sieved in two stacks, remove the first stack from the Ro-
Tap; take the material caught in the pan, and dump it carefully into the top of the
second stack (be sure there is another pan on the bottom of the second stack).
Place the first stack on its now empty pan again.

6. Take a large sheet of brown paper (at least 18” X 18”), crease it in the middle, and
lay it on the table. Then take a sheet of glazed notebook paper (or larger), crease
it and lay it in the center of the large sheet. Now hold the coarsest screen over
the small sheet of paper and carefuly pour out the sand. Then invert the screen
and tap it gently with the heel of the hand. YOU MUST TAP IT DIAGONAL TO
THE MESH OR THE SCREEN WILL BE DAMAGED.

7. On a spare piece of glazed, creased paper, place the balance pan. Carefully pour
the sand from the pile on your two papers, into the balance pan.

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