‘ Land round Melbourne. 233 
simply pulverised it is an excellent manure for the improve- 
ment of sour soil, and is much valued as a plastic material 
for artistic purposes. 
Gypsum is soluble in water to a certain extent only; that 
is, till the water becomes saturated with it, it being found 
by experiment that 1 of gypsum will be dissolved in 400 of 
water. When exposed to a heat of 424° Fahr., the water 
entering into the composition of this remarkable mineral is 
expelled by evaporation, and the gypsum becomes possesged 
of a peculiar antipathy to any combination with water. If, 
however, heated to 318° only, it readily re-unites with water, 
heat is evolved from it, and a dense crystalline mass appears 
as the result of this combination. This (the efficient 
degree of heat) is the secret of producing plaster of Paris 
from the gypsum in its raw or crystallised state. 
In a purely scientific point of view these crystals possess 
' a considerable degree of interest. They occur both in 
clino-rhombical forms and polygonal columns. Twin or 
double crystals, of which there are specimens now before the 
Society, also exist, together with the single ones. These 
remarkable combinations, presenting a varied field of obser- 
vation to the ardent admirer of nature, are so perfectly 
developed that the crystallographer does not readily distin- 
guish or individualise the twin parts of which they are 
formed. m 
These specimens are of a dirty greyish or soapy colour: 
specific gravity equal to 2:40; and their chemical composi- 
tion, calcium 33, sulphuric acid 46, water 21. That these 
are still in course of formation in the manner which I have 
attempted to delineate there can be but little doubt. This 
supposition applies particularly to those places where fissures 
and cavities are formed in the parched mud, by the intense 
heat of the summer sun—the crystalising power having, in 
such spots, abundant room for operation, besides obtaining 
large supplies of sulphuretted hydrogen, derived from the 
decomposition of organic matters on the surface. 
In order to exhibit a more succinct view of the process 
which I have here endeavoured to describe, I have sketched 
out the opposite classification of details. (Vide plate. ) 
In the same basalt formation in other parts of Victoria I 
have found magnesite, opal, carbonate of iron, carbonate of 
lime, carbonate of strontium, mesotype, and steatite (soap- 
stone). The component parts of all these minerals are held 
in solution in common sea water, whence I am of opinion ' 
that they derive their origin from the enormous evaporation 
DD 
