218 Modern Microscopy 



papers, which are placed on the surface of the water. The 

 forams adhere to the papers, which are then carefully 

 lifted off and dried, the specimens being then brushed off 

 into a tube. Many delicate forms, which would almost 

 certainly be broken in the subsequent processes, may thus 

 be obtained in a perfect state. 



The mud remaining in the basin is then washed, a 

 spoonful at a time, by placing it in a sieve of fine silk 

 gauze, through which a gentle stream of water from the 

 tap is kept running until all the fine particles have been 

 removed. The muddy water should be allowed to settle 

 in a bath, and the solid residuum scraped out and thrown 

 away. The sandy residuum left in the sieve should then 

 be thoroughly dried, and is then ready for examination 

 under the microscope, or, if desired, it may be further 

 purified by the floating and rocking processes already 

 described. 



Foraminifera occur in marine fossil deposits of all 

 geological ages, from the Cambrian to post-tertiary, 

 but they are, as a rule, of sparing occurrence until we 

 reach the cretaceous period. The harder chalks and 

 limestones can only be studied by means of thin sections, 

 but the softer chalks, shales, and clays may be broken 

 up by drying the material in small pieces and washing 

 it over a fine sieve in the manner just described. Float- 

 ings are seldom procurable from fossil deposits, owing to 

 the weight of the specimens, which are generally more or 

 less infiltrated with pyrites or other mineral matter. 



Some chalks and shales which resist the disintegrating 

 action of water after being dried may be broken up by the 

 action of a crystallizing salt, which has been absorbed in 

 a fluid state. Acetate of soda has the most rapid action, 

 but very fair results may be obtained with common washing 

 soda. The material, after being broken up into small 

 pellets, is dropped into a boiling saturated solution of the 

 salt, and kept at this temperature for a short time to allow 

 of penetration. The salt is then allowed to cool, and in 



