Vol. 58. | THE MINERAL ANALYSIS OF ROCKS. 165 
specific gravity. A series of beads should be at hand, differing in 
specific gravity by intervals of two or three units in the third place 
of decimals. When one of these is found to float above a zone but 
not far removed from it, and another similarly below, the difference 
in vertical distance between each and the centre of the zone is 
directly proportional to the difference in specific gravity, which may 
thus be calculated. This determination having been made, the 
actual separation of the minerals of the rock may next be under- 
taken. Before I proceed to describe this, I may call attention to 
the important information which is incidentally afforded by the use 
of the diffusion-column ; for it not only provides us with a know- 
ledge of the specific gravity of the component minerals of a rock, 
but shows at a glance what minerals are absent as well as which 
are present, thus supplementing in a ready manner the knowledge 
acquired by an examination of thin slices. Orthoclase, which might 
readily be overlooked when present in small quantity in a basic 
rock, thus frequently reveais its existence, while its absence is as 
often confirmed. 
In describing the process of separation, we may suppose as an 
instance that we are provided with a rock containing orthoclase 
(sp. gr. 2°56), quartz (sp. gr. 2°65), andesine (sp. gr. 2°67), biotite 
(sp. gr. 3°1), pyroxene (sp. gr. 3°3), and magnetite. The fluid used 
for the first separation should then have a specific gravity of 2-885, 
the mean of that of andesine and biotite. The rock may by its 
means be divided into two portions, the heavier of which will 
contain the biotite, pyroxene, and magnetite; the lighter the ortho- 
clase, quartz, and andesine. If, however, one of the minerals 
present a wide range of specific gravity, the line of separation 
between it and its neighbours should be drawn midway between its 
extremes. Thus in the case of biotite, which may range from 2:9 to 
3°1, we should divide it from andesine by fluid of sp. gr. 2°78. The 
quantity of the rock taken for analysis should have been previously 
determined by weighing. The sum of the weights of the separated 
portions should be compared with this, and the loss distributed. Each 
of the two portions may be treated in the same way, the quartz 
and orthoclase being separated from the andesine by fluid of 
specific gravity 2°66, the orthoclase from the quartz by fluid of specific 
gravity 2°605, and so on for the other constituents. By making 
the first separation in the middle of the series, we are able to 
operate simultaneously upon the resulting portions and thus to save 
time. 
The separating apparatus (figured on p. 166) that I have found 
most convenient in practice consists of a wide glass tube, expanded 
into a pyriform enlargement at its upper end. It is fitted with well- 
ground stoppers above and below; and with a third stopper carried 
at the end of a long glass stem, both stopper and stem being 
traversed by an axial canal of narrow, but not capillary lumen. 
This stopper fits into the separator at about the commencement of 
the pyriform enlargement. In making a separation, the. lower 
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