26

Earths in which the percentage of lime and other impurities is con-
siderable are usually of a grey to cream yellow colour and are granulated
in texture. An earth which from its analysis was calculated to contain
36 per cent of lime carbonate and 50 per cent of anhydrous magnesium
carbonate (see Table IV, analysis 6) seems to be made up nearly wholly
of rounded, carbonate grains 0-002 to 0-005 millimetres in diameter, that
are undoubtedly anhydrous carbonates, possibly dolomite. There are also
fine, needle-shaped crystals of high birefringence, probably brucite, and
a cloudy looking, very finely divided substance, the nature of whith was
not determined, but part of which may be amorphous silica. The com- -
paratively small proportion of brucite (Mg (OH).2) demonstrates that most
of the magnesium exists as the anhydrou’ carbonate (Mg COs), either as
magnesite associated with calcite, or combined with calcium carbonate
as the mineral dolomite! ((CaMg)Cos).

Earths made up nearly or wholly of gypsum resemble in the hand
specimens those containing relatively large amounts of lime. Under the
microscope the gypsum crystals were recognizable, but anhydrite, which
is probably present, could not be determined because of the fineness of
the material.

The white hydromagnesites are fairly dense; the earths with more
impurities are granulated. The granulated, in very many cases, show well-
defined, thin beds of =; to inch in thickness, lying flat but with a slightly
wavy outline. The same banding is observable on freshly broken surfaces
of the hydromagnesites. Between the bands are flattened cavities-with
rounded and curved surfaces.2 In places near the groundwater level the
impure earths are cemented into compact masses. :

Relation of Hydromagnesites and Impure Earths.

The nearly pure hydromagnesite when present in the larger deposits,
lies at the surface in a layer of from about 1 foot to 23 feet or, in rare cases,
43 feet in depth. Under it is almost invariably a layer of cream-coloured
or yellowish earth carrying a fairly large percentage of lime, silica, and ~~
other impurities. The combined thickness of the two layers varies from
about 4 to 6 feet. The division between the two is as a rule fairly sharply
defined by a wavy but nearly horizontal surface. Near the edges of the
larger deposits irregular, lens-shaped layers full of brown sand _ lie between
these two zones. Where, in the same deposit, both white hydromagnesite
and impure earth outcrop at the surface, the top of the white earth is at a
slightly higher elevation than the neighbouring surfaces of impure material.
Below the cream-coloured, impure layer is generally a dirty white earth
full of sand and clay, cemented in many cases to a hard mass, and below
this is more sand or clay. At Watson lake the succession described above
seems in places to be repeated so that a white layer underlies a yellowish

layer. In places the underlying clays contain numerous, small, freshwa er.
shells. -

1The formation of dolomite—if, indeed, this be dolomite—under the conditions in which these earths originated
would be of great interest geologically. See K.A. Redlich in Fortschr. der Miner. Kirst und Petrog. Deutsch
Miner. gesells. Band IV, pp. 9-42, ‘Die bildung des magnesits und seine naturliche Vorkommen.” Attempts
by ee. ordinary micro-chemical tests to discover whether dolomite or calcite was present in this earth did not
succeed.

*The texture and structure of these beds, and especially of the calcareous tufas, pages 49 and 51, resemble
the structures called stromatoliths by Kalkowsky, see Zeitsch der Deut. Geol. Ges. Band LX, 1908, pp. 68-125.