to grind a series of smooth surfaces at pre-determined in- 
tervals. It is possible to make from 250 to 500 peels per 
inch although such frequency is rarely desirable. The ac- 
tual amount of fossil in the peel is from one to three micra 
in thickness. In order to guarantee uniform intervals be- 
tween peels, the specimen must be calibrated. Small or 
fragile specimens should first be imbedded in plaster or 
W ood’s metal. The latter is excellent but, unfortunately, 
expensive since several pounds are needed to imbed even 
a small specimen. The actual grinding is done with a fine 
or medium carborundum powder. Number 320 will do, 
600 is better, and any size intermediate will give good 
results. Although grinding may be done by hand, a re- 
volving lap is highly reeommended for exact work. The 
abrasive powder is placed upon the lap and then moist- 
ened with water. The lap is set in motion (or the speci- 
men is rotated by hand in one direction only) and the 
surface is smoothed and ground to the desired distance. 
The lap must be continuously moistened. If abrasive 
powders of different fineness are used, coarse should not 
be permitted to contaminate fine, otherwise deep 
scratches will mar both the smoothed surface and the 
lap surtace. 
Etching. Etching is the process by which the rock 
matrix is removed, with acid, from the carbonized struc- 
ture. It is this process which may lead to success or fail- 
ure, because careless etching destroys the cellular detail 
and trays the outline of the cell walls. It is necessary to 
use not only different kinds of acids but also different con- 
centrations. Ordinary calcified specimens may be etched 
with a 1% solution of hydrochloric acid for 30 seconds. 
Dolomitic specimens which contain magnesium carbon- 
ate may require a 10% solution for one minute. 
Silicified specimens must be etched with commercial 
hydrofluoric acid (499%) for three minutes, although sub- 
[71] 
