Apr. 1899. A Fossil Egg from South Dakota — Farrington. 197 



likewise only shelis, which have been preserved by reason of their 

 thickness. Neither of these occurrences are, therefore, cases of true 

 petrifaction. At first thought, an egg of the sort here described 

 may seem too perishable for preservation by a process of true petri- 

 faction. It is difficult to understand how, in such a mass as an egg, 

 petrifying liquids could pass to and fro, removing particles of organic 

 matter and replacing them by particles of silica, in the way that it is 

 generally understood that petrifactions usually take place. On further 

 consideration, however, the natural petrifaction of an egg need not 

 seem to be an impossible phenomenon. If covered as soon as de- 

 posited, by mud or earth, as it is likely to have been in this region, 

 its substance might endure for months or years. Or, the process of 

 petrifaction might have begun at once, since the present chalcedony 

 veins of the region show that circulating siliceous waters are abund- 

 ant there. 



Given conditions of this sort, I believe that petrifaction could have 

 gone on by a process of endosmose and exosmose similar to that 

 believed by M. Forster Heddle* to produce the formation of agates. 

 As the cases seem so similar in their conditions, his theory may be 

 quoted in full: ''We have now a cavity slightly lined with chalce- 

 donic matter, containing, within, water more or less pure, while with- 

 out (that is, outside the now double skin, delessite and first layer), 

 we have a strong solution of colloidal silica constantly supplied. 

 Endosmose and exosmose are set up with resistless force. The strong 

 solution finds its way through the two or any number of increasing 

 skins; the weak water is forced out through the point of infiltration, 

 and so in its passage out thins all the successively deposited layers 

 at that place. By the continuous flow of colloidal silica (held in 

 solution by liquid) through the already coagulated or deposited layers, 

 continuous coagulation of the silica in the yet hollow agate, and con- 

 tinuous extrusion of the residual water, we have the ultimate filling 

 up of the cavity, and a solid agate formed." The parallelism of con- 

 ditions in the two cases is so apparent as to need no emphasis. The 

 shell of the egg and its lining membrane furnish the "skin," the 

 albuminous or watery substance within the egg the weak solution, and 

 the circulating siliceous waters known to abound in the region the 

 strong solution of colloidal silica. Or the positions of the latter may 

 have been reversed, the thicker liquid having been within and the 

 thinner without. In either case a transference would take place. 

 While I cannot say that Professor Heddle's theory, that agates have 

 been formed in this way, is altogether the adopted one, the stages of 



* Nature, Vol. XXIX, p. 419. 



