BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [22] 



maybe a few good specimens of Polycystina. Each layer of clay, as deposited by 

 its specific gravity, has now been examined, and most of the fossils are contained in 

 some or possibly two of them. Nineteen- twentieths of the original sample of clay 

 have been washed away, and in the selected one-twentieth that remains there may 

 be one fair fossil to 100 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 lamime. 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 fossils 

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

 of clay. 



Chalk. — Foraminifera, coccoliths, rhabdoliths, with an occasional radiolari an (Poly- 

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

 be freed from the rock by washing 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 radio- 

 laria will promptly carry them to the bottom, and they can be partly separated 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 and rhabdoliths, 

 which largely give to the water its milky appearance, will be lost. They are very fine 

 and very light, and some of them will remain suspended in a 4 ounce beaker of Avater 

 for several hours. They can be separated from the other material hj repeated wash- 

 ings and decantations, so as to make almost pure mountings, but the resting time 

 between decantings must be from one-half to three-quarters of an hour.^ 



Limestones. — Entomostraca, and other small organisms in which the valves are 

 united, may also be obtained in a perfect coudition.from this class of rocks, by pound- 

 ing fragments of the fossiliferous material with a hammer within the circle of a small 

 iron ring or '' washer," one-eighth of an inch in thickness. As the rock is crushed 

 by the blows of the hammer the organisms jump out of the matrix, but are retained 

 within the bounds of the ring, Avhich also answers as a gauge, preventing the ma- 

 terial from being broken too small. The pounded rock is afterwards washed free 

 from dust, dried and searched as above directed. - 



DIRECTIONS FOR HARDENING SOFT FOSSILS. 



Fossils from friable sandstone, ferruginous sandstone, soft clays, oi^ 

 marls, if not hardened, will usually soon be ruined. One method of 

 hardening is to warm the fossils, then dip them into hot, thin glue water. 

 The glue water must not be too thick, otherwise the specimens will have 

 a decided gloss and stick to the trays in moist atmospheres. Warming 

 the fossils before dipping assists the glue water in penetrating deeper 

 into the rock, and it also guards somewhat against the glue gloss. 

 However, a more desirable and quicker, though somewhat more expen- 

 sive, method is the shellac process. All that is here required is to have 

 a very thin solution of white shellac, in which fossils are well soaked 

 and then laid aside to dry. Such hardeued fossils will never stick to 

 moist fingers, as is the case with glue-soaked specimens. Dr. W. H. 

 Dal], of the Xational Museum, employs the shellac process entirely to 

 harden all soft Tertiary marl fossils. 



^Woodward and Thomas, The Microscopical Fauna of the Cretaceous in Min- 

 nesota, Final Report Geol. Nat. Hist. Survey Minnesota, Chap. II, pp. 25-27, 1893. 

 2GEIKIE, p. 673, 1893. 



