OF DENISON UNIVERSITY. 
123 
be optically examined, though there are many opportunities for error. 
It is recommended that rocks of a rather finely granular structure be 
examined in this way with reference also to the specific gravity. A 
fragment of suitable size is reduced to a powder, the fineness of which 
may vary with the size of the granular elements in the sample. The 
powder is then assorted under water by agitating repeatedly in a bottle 
and hastily pouring off the fluid, leaving the part first to settle and re- 
peating the process as often as any separation is possible. Denser 
fluids (See Hussak Anleitung, p. 51,) may be used wdth advan- 
tage in some cases. By mounting different parts of the powder 
thus sorted separately or under separate covers upon a single glass 
slip, interesting qualitative and even approximate quantitative results 
may be obtained. 
Comparatively few minerals or aggregates are sufficiently transpa- 
rent to permit of optical examination by transmitted light. A few of 
these, as mica, gypsum, calcite, dolomite, etc., are adapted for study 
without other preparation in any way. In cases where the cleavage 
is irregular and does not permit the breaking out of tabular plates it 
becomes necessary to mount the irregular fragments in a highly refrac- 
tive medium, such as balsam, between glass slips, and thus to elimi- 
nate as far as possible the dispersion and irregular refraction. Even 
then the results are often unsatisfactory. Sands and other fragment- 
ary rocks may be examined by the following method, suggested by 
Thoulet. 
The powder or sand is mixed with about ten times its volume of 
zinc oxide, then silicate of potassium is stirred in till the whole assumes 
the consistency of a stiff broth. This, while still soft, may be pressed 
into a mold, formed from a section of a glass tube glued to a slip. 
When set, the resulting cylinder is removed and fastened to a slip of 
thick glass and ground in the way described beyond for compact 
rocks. 
When a rock or mineral is not transparent and does not admit of 
breaking into sections of suitable thinness with the hammer, it becomes 
necessary to cut or grind a section of sufficient transparency and then 
mount it between glass in Canada balsam. Before beginning this 
somewhat tedious process the student should provide himself with the 
following outfit : ( i ) A number of thick squares of plate glass about 
one half inch thick and two inches square, these being ground on the 
edges in order to avoid cutting the fingers ; (2) several dozen slips of 
