26 THE PALEONTOLOGY OF MINNESOTA. 



[Preparation of shale and chalk. 



balsam will be hard when cold. As these "trial slides" are seldom of any value, 

 it is not necessary to use cover glasses if the balsam is hardened, as above directed. A 

 careful examination of these slides under the microscope with a good \ or \ inch 

 objective will decide as to the value of the material under observation, and if it 

 proves to be only sand, pour it all out, wash the bottle, and again try the same 

 process with another sample of clay. But if the slides show a few good fossils, the 

 next step is to separate them as much as possible from the mass of sand*, etc., with 

 which they are associated. In this as in the first washing, specific gravity will do 

 most of the work. Pour off most of the water and put the shells, sand, etc., into a 

 4 oz. beaker (or glass tumbler), wash "out the bottle, fill the beaker about f full of 

 water, and after it has rested ten minutes, pour f off the top through a glass funnel 

 into the bottle, repeating this 5 or 6 times. As in the first washing, mount and 

 examine a few slides from the material at the bottom of the bottle, mounting and 

 preserving slides, if found to be of value. If nothing of value is found, pour out the 

 contents of the bottle, and fill up again as before from the beaker, after five minutes 

 rest repeating these washings and examinations at shorter resting intervals of say 

 three, two, and one minute, or less, until nothing but the coarsest sand remains in 

 the beaker. In that there may be a few good specimens of Polycystinn. Each layer 

 of the clay, as deposited by its specific gravity, has now been examined, and most of 

 the fossils are contained in some one or possibly two of them. Nineteen-twentieths 

 of the original sample of clay have been washed away, and in the selected one-twen- 

 tieth that remains there may be one fair fossil to one hundred grains of sand. 



Shale. The fossil contents of most of the softer shales can be secured by breaking 

 up the specimen with a pair of strong pliers, crushing the shale while under water 

 and edgewise of its laminae. This will free many of the fossils without breaking 

 them ; then boil the firmer parts of it for a few minutes (or longer if the material 

 requires) in "a rather strong solution of washing soda, and wash and separate the 

 fossils from the fine shale, sand, etc., by repeated decantations, as directed in the 

 treatment of clay. 



Chalk. Foraminifera, coccoliths, rhabdoliths, with an Occasional radiolarian 

 (Polycystina), of which the "farmer's chalk", or soft limestone, is largely composed, 

 can be freed from the rock by washing off the surface of a clean piece of it with a 

 rather stiff brush while under the surface of the water in a bowl or basin. The 

 water will soon become as white as milk. The specific gravity of the Foraminifera 

 and Radiolaria will promptly carry them to the bottom, and they can partly be sep- 

 arated from the sand, etc., by repeated washings, decantations, etc., as directed in 

 the treatment of clay ; but unless great care be taken in this washing, the coccoliths 



