Under A1
we are only immediately concerned with some forms of Gypsum, which come
under (c). During past geological times there were in some areas great
closed lakes similar to the Dead Sea at the present time ; the rivers
flowing into these brought down many salts in more or less dilute
solution ; now it is at once obvious that if a lake persists with no
outlet, the evaporation from its surface must be sufficient to account
for all the water brought to the lake by the rivers flowing into it
(with the exception of a small quantity which may intrude into the
surrounding rocks). Thus, as the water evaporates the solution of the
various salts is concentrated, and some of the salts finally deposited.
The Gypsum probably results from the double decomposition of magnesium
suljmate and calcium hydrogen carbonate, both contained in the water.
It may also arise by the concentration of estuarine or brackish water
in shallow lagoons. Once it is disseminated in scales throughout the
mud at the bottom of the lake, we can more easily account for the
subsequent segregation into masses which has occurred in most known
gypsum-beds; such segregation would come under the head A3 (b).
The next group we are concerned with is A3 (a) dealing with those minerals which have been produced by alteration
