220 Microscopical Examination of Rocks. [ April, 
of alteration; it is therefore necessary to choose fragments both 
from the fresh and decomposed parts, as thin sections of the lat- 
ter ones are just as important and instructive as those taken from 
parts in which no visible change has occurred. Usually such — 
decomposed rocks are very fragile and crumbling, because their 
crystalline structure or their cementing medium has been 
destroyed by the process of decomposition. Before undertaking : 
to grind them their firmness must be restored. The fragments 
of such decomposed rocks are at first well dried and then placed 
in liquid Canada balsam, which is exposed on an iron plate toa 
gentle heat till it has filled and penetrated the pores and fissures 
of the rock. They are then separated from the Canada balsam 
and laid upon the warm iron plate till the balsam proves to be 
hard when cooled down, and are then capable of undergoing the 
process of slicing or grinding. i 
Apparatus.—It consists in its most simple form of some plates 
of iron and glass about eighteen inches square, to allow a full 
stroke with the arm during the grinding process. They haveto — 
be perfectly even and flat. Emery of various degrees of fineness 
is required to be used on the plates with an addition of water. — 
Fragments of rocks are ground by hand and treated as will be 4 
described in the following lines: 
In the New York State Museum of Natural History at Albany, 4 
where a large number of rocks and fossils have to be cut and i 
prepared for thin sections for illustrating the Palaeontology of 
New York, the slow process of grinding by hand had to be 
abandoned, and a small boiler and steam engine of two and 
a-half horse power were purchased for the purpose of using 
steam-power for the grinding and slicing operations. The grind- 
ing apparatus is formed by two circular iron plates (A and B) of 
eighteen inches diameter and one-half, inch thickness, which 
rotate horizontally with a velocity of about 300 revolutions 4 
minute. The slicer which is used to cut the rock in certain — 
directions, or to separate from it small slices to be ground after- a 
wards, is a thin copper or steel disk of twelve inches diameter, ss 
and turns also horizontally with the same velocity as the grinding 
plates. For rocks above hardness 5, another slicer of steel 15 
used, which turns vertically, and which is fed with diamond 
powder moistened with oil. 
The whole apparatus may be run at the same time, an 
d is 
