ON OOLITES AND SPHERULITES 603 
solid; that the spherical shape of the grains is due to the tendency 
of the droplets forming during this process of separation to coalesce; 
and that the difference between radial and concentric structure 
depends on the amount of other substance thrown out simul- 
taneously with and mechanically enmeshed in the growing struc- 
ture. 
The chief value, at present, of this interpretation of the origin of 
odlites to the geologist lies in the fact that it gives a new direction 
to future work. The questions to be answered in each case are: 
1. What factors determined the colloid dispersion of the salt 
and what was the medium .of dispersion ? 
2. What caused the separation of the colloid from the dispersion 
medium ? 
3. What made possible the suspension of the growing spherite ? 
The chemical geologist will have to decide in each case whether 
the factors involved in the first two questions were physical (for 
instance, the presence of protective colloids) or chemical (for 
instance, the nature and quantity of other substances in solution) 
or biological (for instance, the action of bacteria). 
INTERPRETATION APPLIED TO SEDIMENTARY OOLITES 
Shape of grams due to growth in suspension.—The last question, 
on the other hand, is of special interest to the stratigrapher. We 
were accustomed to think that calcareous or limonitic odlites owe 
their spherical shape to constant rolling on the sea (or lake) bottom. 
While it cannot be said that such an origin is impossible, there is 
experimental evidence which raises doubts in my mind as to whether 
agitation of the suspension or dispersion medium would ever allow 
of the formation of such structures. Besides, there are now very 
few cases left in which such an interpretation might seem necessary. 
The alternating layers of silica and of carbonate of the pisolites 
form while the spherites are being carried up by the current of the 
flowing spring water. The layers of hailstones form as they fall. 
Limonitic odlites form in the gels of organic iron salts on the bottom 
of Swedish and Finnish lakes.t Odlites of bauxite and of limonite 
« For references see F. Beyschlag, J. H. L. Vogt, and P. Krusch, The Deposits of 
the Useful Minerals and Rocks, translated by S, J. Truscott (London, 1916), IT, 982. 
