74 On Enhydros found at Beechworth. 



under the microscope, under an amplification of 90 

 diameters, it is found that besides this impress of the super- 

 ficially attached bloom, the horny chalcedony shows also 

 distinct and beautiful indications of regular crystalline 

 structure, — a net- work like a geometrical carpet pattern 

 extending over the whole field. 



Now it has occurred to me as possible that this crystal- 

 line structure in the chalcedony may afibrd the key 

 to an explanation of the formation of these paradoxical 

 enhydros, and if speculation, founded however on obser- 

 vation of physical facts, be permissible, I should like 

 to add a few words hinting what these crystalline 

 demarcations have suggested to my own mind. The 

 enhydros have this in common with agates, namely, an 

 exterior of silica in the horny state, for the most part 

 colloidal silica, and an inner laj^er of quartz crystals. The 

 successive layers in both instances have been deposited from 

 without inwardly ; that is to say, the chalcedony or agate 

 first, the interior quartz crystals last ; and 'in the case of 

 agate each layer of agate successively from the exterior 

 layers to the interior ones. The enhydros also agree with 

 agates in showing points of infiltration, some of the 

 enhydros leaking at one of the solid angles. 



Now, with agates, it is at least conceivable that water 

 charged with silica and depositing it in a cavity in the 

 amorphous form, would, if the supply were kept up, con- 

 tinue so to deposit it, layer on layer, as long as a full water- 

 way existed ; but when eventually by the deposit of this 

 chalcedony or agate, this way is stopped up, thereafter 

 any silica in solution entering into the central cavity 

 must be dialysed by liquid difiusion through the mois- 

 ture filling the pores of the amorphous silica. For 

 this to take place a solution of silica in both the 

 crystalline and amorphous states is requisite ; but this 

 necessity is in agreement with the composition of chalce- 

 dony, which is generally admitted to be an intimate 

 mixture of amorphous and crystalline silicic acid. The 

 crystalline character is masked by the colloidal — masked, 

 but not therefore necessarily altogether suppressed. This, 

 indeed, is the point upon ivhich my suggestion hinges. 



If the crystalline texture observed in the thin laminge 

 from the outer layers of these enhydros is due to crystalline 

 quartz, we may then ask whether chalcedonies do not differ 

 in the proportions of the amorphous and crystalline consti- 



