1891. | DERBY—ON ACCESSORIES OF ROCKS. 199 
sufficient size and capacity to afford as large a residue as will, in most 
cases, be required, and can readily be carried on excursions in a cloth 
sack slung from the shoulder. In such a batéa a quantity of crushed 
or decomposed rock, equal in amount to a large sized hand, specimen, 
can be treated at a single operation, which should generally afford 
sufficient heavy residue for a considerable number of microscopic 
slides. If material is scarce, a much smaller volume will usually be 
found to give a satisfactory result unless the accessories are ex- 
tremely rare, as fragments (which can be chipped from a museum 
specimen without injury) representing the bulk of a butternut will give 
enough for at least one slide. 
The knack of washing is readily acquired, and with a litile exper- 
ience one soon learns to vary the process according to the character of 
the material. In washing a decomposed granite, for example, the first, 
process is to thoroughly disintegrate the mass and to get rid of the 
clayey portion by kneading and stirring it under water with the hand, 
pouring off the suspended clay with frequent changes of water. When 
sufficiently free from clay to permit the granular portion to move freely 
in water, a vigorous shaking from side to side with a slight circular 
motion brings a layer of the coarser fragments of quartz and feldspar 
to the top and, after pouring off the water, this is scraped off with the 
hand. After repeating this process till the remaining sand is of com- 
paratively uniform grain, a circular motion is given to the batéa which 
brings a considerable portion of sand into suspension in the water, 
and permits the heavier and finer grains to settle towards the centre, 
while the lighter and coarser ones tend towards the surface. By a 
dexterous jerk the water and moving sand is thrown to the side and, 
after pouring off the water, the outer portion of the trail of sand on the 
sloping side of the bateéa if scraped off with the hand. After repeating 
this process until the volume of sand is reduced to about a teaspoonful, 
it will be noticed that the white color given by the predominance of 
quartz becomes tinged with black and red through the concentration 
of the iron minerals and garnet. When, at about this stage, the motion 
is sufficiently vigorous to set the whole mass swirling about the side of 
the batéa and is then gradually slowed down, the heavy concentrate 
may be seen to settle together in a very pretty manner, in a mass at the 
apex of the batéa, while the quartz swings about in the water on top, 
When a little farther reduced the behavior of the different minerals 
according to their specific gravity is most beautifully seen. On 
throwing the’sand out into a trail, it will be found to be transversely 
streaked with different colors, according to the arrangement of the 
