302 THE FOEAMINIFEEA 



turbid, and theu the residue can be turned out for 

 final cleansing. This may be done by taking a 

 corked bottle, in which we place the chalky sand, and 

 with the merest trace of water ; the bottle is then 

 shaken vigorously, so that the particles may be cleaned 

 by a certain amount of friction. If our specimen is 

 well selected, the result will be a sand largely 

 composed of the shells of Foraminifera. Another 

 method, and perhaps a safer one for the shells, is 

 to take a piece of soft chalk and with a tooth- 

 brush briskly rub it under water until a fine sandy 

 residue falls down. 



On a larger scale the actions of frost and rain 

 effect what we have here been trying to do ; for it 

 is not uncommon to see in chalk districts a fine 

 powdery material lying at the foot of the dip slope, 

 where the rain drains off into runnels, which often 

 consists almost entirely of foraminiferal shells. 



In searching for Foraminifera in the harder rocks 

 we must resort to a different method of investigation ; 

 for the foraminiferal limestones of the Carboniferous 

 system, and also those of Eocene age, are often 

 extremely hard, and resist all attempts to disintegrate 

 them by ordinary methods such as we have just 

 described. These hard fossiliferous rocks are shced 

 in the usual way in cutting thin rock-sections, by the 

 lapidary's slitting machine. The slice is then surfaced 

 by grinding on fine emery, and, if it be a limestone, 

 faced on a whet slate. The prepared surface is then 

 cemented to a glass slip by using Canada balsam, and 

 ground down by successive grades of emery powder 



