300 THE FOEAMINIFEEA 



familiar clays and marls which may thus be treated 

 with satisfactory results we may mention the Lias 

 Clays, the Gault Clays and Marls, the London Clay, 

 Bracklesham Clay, Barton Clay, and the Pliocene 

 Clay of St. Erth. 



The materia] chosen to be worked upon for its 

 Foraminifera is broken up into pieces about the size 

 of a walnut, slowly and thoroughly dried in an oven, 

 and dropped, preferably when warm, into a vessel of 

 cold water. The material thus treated should be left 

 to remain undisturbed until the whole of the material 

 has broken up into a fine silty mud, which condition 

 may be tested by raking up a sample with a fork. 

 This precaution has to be taken on account of the 

 tendency of the clay to clot together when touched 

 with the lingers. When the clay is thoroughly dis- 

 integrated, forming a more or less fine mud, we may 

 commence to wash away the finest argillaceous por- 

 tion. The decanting should be done with care, so as 

 to run no risk of pouring off the shells with the 

 mud. The washing is repeated until the water is 

 no longer turbid. For the final cleaning of the 

 residue, which often takes a little extra time if the 

 material is close and refractory, we must pour off as 

 much of the water as possible and vigorously work 

 the vessel with a rotary motion until the residue looks 

 of the consistence of cream. Then we pour on water 

 to fill the vessel, allowing the material to subside 

 before decanting. This should be done repeatedly 

 until no more fine clay can be separated, and the 

 residue looks clean and sandy. Should some clay- 



