94 
RECORDS OF THE AUSTRALIAN MUSEUM. 
for the uniformly flat and even character of the surfaces, and the 
perfectly straight edges in which those surfaces meet. Mr. 
George Foord’s theory, that a mixture of colloidal and crystalline 
silica might have a tendency to assume a definite form (that of 
plates) on deposition is obviously insufficient to account for them, 
in view of the fact that some of the specimens from America are 
composed entirely of quartz. 
A more probable explanation appears to the writer to be this, 
that their geometrical form is due to the deposition of chalcedony 
or quartz on the walls of cavities formed by the intersection of 
tabular crystals of calcite, the latter having been afterwards 
removed in solution leaving the enhydros free. The thin septa 
frequently observed in them are formed in the same manner, the 
laminse of calcite being very thin, and the complete specimens 
in reality a combination of two or more single ones. The 
occurrence of numerous plates of chalcedony with the enhydros 
is merely what one would expect, they are, no doubt, broken 
fragments of similar bodies which were too thin and fragile to retain 
their original form after removal of the calcite. The exterior 
surfaces of the enhydros would of course reproduce in an inverted 
manner those striations, markings, etc., which happened to be 
existent on the surface of the calcite lamina), and might therefore 
lead to the supposition that the chalcedony itself partook of a 
crystalline character. 
On the above assumption the angles between the surfaces of 
the enhydros must be those between the laminae of calcite, and 
some among them would therefore be the same as those known 
to exist between corresponding surfaces of calcite tables in twin 
position. From among the numerous angles so formed, several 
were found to agree, as closely as could be expected from the 
rough means of measurement at my disposal, with the known 
angles 127° 29^', 52° 30£', 90° 46', and 89° 14'. 
The above view of their formation has been further strengthened 
by my finding among the numerous mineral specimens in the 
Museum Collection one in which thin tables of calcite intersect 
forming geometrical cavities, the walls of which have received a, 
very thin coating of silica. This specimen may therefore be 
considered as shewing the enhydros in an initial stage of forma- 
tion. Casts in gelatine taken of a few of these cavities gave 
forms very similar to those of some of the enhydros. 
The latter bodies then, if the above explanation be the correct 
one, are casts of cavities : and a complete series of them, placed 
in the position in which they were originally formed, would con- 
stitute a mould of those calcite crystals on which the chalcedony 
and quartz were deposited. 
16th September , 1895. 
