CRETACEOUS FLORA 25 
Preparation of clay.] 
than any other strata with which they are associated, the great sandstone reservoirs 
not excepted ; and careful estimates based on chemical investigations show that if 
the shale series is counted at the low average of 1,000 feet in thickness for its entire 
area (it is over 400 miles long and 10 to 30 miles wide), it would yield over ten 
million [10,000,000] barrels of oil to the square mile.” While this oil must largely 
be separated from the shale by distillation, which is now being done at the rate of 
some 20 gallons to the ton, these estimates are truly bewildering. 
Most of the material used in these investigations was sent to us by Prof. N. H. 
Winchell, state geologist of Minnesota, and was of almost every kind and variety 
that a geologist would naturally collect for microscopical study, and they required 
almost as many different methods of treatment to free from them their microscopical 
organisms in proper condition for examination under the lens, as there were samples 
of material. While we cannot, of.course, give all of the various experiments 
necessary to ascertain the chemical character of the materia] and of its organic con- 
tents, before deciding whether its reduction, so as to preserve its fossils in their best 
condition for examination, would require the use of rain water, nitric or other acids, 
soda, caustic potash, &c., we will give a few general directions that may be of some 
assistance to beginners. 
One of the first essentials is that all glassware, pipettes, &c., designed for this 
use be absolutely clean, and that only river or rain water, recently filtered, be used ; 
otherwise you will probably find on your slides many beautiful organisms that do not 
belong to the substance under examination. 
Clay. In preparing most of the samples of clay, we would put about one ounce 
of the material, and the same amount of common washing soda, into a druggist’s 
two-quart clear-glass packing-bottle, not over +4 filled with water, and let it remain 
12 to 24 hours, frequently shaking the bottle so as to thoroughly break up the clay. 
Now fill the bottle with water, and after 25 minutes carefully pour off the upper } of 
it. Again fill with water, and in 25 minutes decant as before ; repeating this at 25 
minute intervals until the upper ? of the water in the bottle, after a 25 minute rest, 
will be nearly clear. A large amount of the fine sand, clay and the soda, has by this 
process been washed away, and the action of the soda has broken up the clay and 
removed most of the adhering material from the fossils. Now mount a few micro- 
scope slides from the residuary sand, etc., at the bottom of the bottle, by taking up 
with a pipette (a piece of small glass tubing makes the best pipette) a small amount 
of the material ; scatter very thinly over the middle of the slides; dry them thor- 
oughly over an alcohol lamp, or in some better way, and, while hot, cover the dry 
material with a few drops of Canada balsam, keeping the slide quite warm until the 
