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 probaV)le 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 

 laminae of calcite being very tliin, 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 laminse, 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 laminse of calcite, and 

 some among them would tlierefore be the same as those known 

 to exist between corresponding surfaces of calcite tables in twin 

 position. From among the numei'ous 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° 29h', 52° 30^', 90° 46', and 89° U'. 



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. 



