Apr. 1899. A Fosst. EGG rrom SoutH DakoTa—FarRRINGTON. 197 
likewise only shells, 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 usuallytake 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 belreve 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 1s 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. X XIX, p. 419. 
