
THE COLLECTION OF MINERALS 
The suite of quartz specimens beginning in Case 4 is exceptionally 
fine. At the head of the series will be found the sharply defined, 
brilliant, transparent crystals which are familiar to most of us. These 
are characteristically six-sided with prisms of varying length and occur 
as single crystals or in groups. In addition to the specimens showing the 
wide range of crystal habit, attention is particularly directed to the 
examples of phantom quartz and capped quartz, which illustrate the 
effect of a change in, or the temporary arresting of the action of the silica- 
depositing solution, the result in both cases being the production of a 
quartz crystal around a similar and previously formed one. Small 
amounts of such impurities as titanium, manganese and organic matter 
produce respectively the colored varieties, amethyst, rose quartz and 
smoky quartz (Case C). 
There are also many examples of quartz enclosing other minerals, 
such as sagenite, enclosing slender needles of rutile, and cat’s-eye 
quartz, in which the silica solution has surrounded and imprisoned 
hair-like fibres of asbestos. The massive forms of quartz (Case 5) 
are distinguished by their entire lack of outward evidences of crystalliza- 
tion. Here the mineral assumes rounded outlines, similar to and 
produced in much the same way as the icicles of frozen water or the 
stalactitic deposits which are formed by the dripping of mineral solu- 
tions in a cavern. Beginning with chalcedony, which well illustrates 
the deposit of quartz from a silica solution of uniform composition, the 
series shows a great variety of agates, in which the layers of differently 
colored quartz have been produced by a change in the amount and char- 
acter of the coloring impurity in the silica-depositing fluid. Considerable 
amounts of iron and clay give rise to the opaque, massive varieties, 
jasper and basonite. 
Opal is a hydrated oxide of silicon, that is, it has the same chemical 
composition as quartz except that it contains a varying percentage of 
water. Among the many varieties of opal in Case 5, the one which 
appeals most strongly on account of its beauty is precious opal. The 
brilliant and varied play of color which is a well-known characteristic 
of this mineral is supposed to be caused by incipient cracks in the mass 
of the stone. These reflect back the light in the same way as the film 
of a soap bubble or of oil spread on water. Both massive quartz and 
opal under favorable conditions replace the woody tissue of trees, produc- 
13 
