ever,
occurring in sandstones and limestones of Tertiary age, doubtless were
formed in place, and owe their deposition to the circulation of
siliceous waters through the rocks.
The
distribution of gems through a rock or gravel matrix is not usually
uniform. The gems more commonly occur in pockets, so called, the
location of which seems to be governed by no law as yet discovered.
Where crystallization of minerals has taken place about a fissure or
open cavity, the minerals are more likely to be clear and free from
inclusions than where formed in the mass of the rock itself.
