ROCKS FROM NORTHERN AND CENTRAL CHINA. 389 
between layers of more clayey constitution, which meet about its margin. The layers 
may retard or prevent diffusion, and the lenses may come to have a uniform content 
of lime, which is higher than that of the surrounding sediment. 
Another suggestion is based on the capacity of inorganic and organic colloids to 
absorb carbonate of lime. Among such colloids are hydroxides of iron and alumina, 
organic acids, and organic gelatins. ‘These are all present, it may be presumed, in sedi- 
ments which result in calcareous, ferruginous, and fossiliferous shales. Ifa dilute solution 
of carbonate of lime, which is alkaline, comes in contact with such a colloid, a certain 
portion of the carbonate is absorbed.* 
If it be supposed that the absorbent mass is limited vertically and horizontally (and 
any other distribution seems improbable) we have a local condition capable of concen- 
trating lime carbonate in a manner to promote local induration. 
Organisms, a chief source of carbonate of lime taken from sea-water, play a part 
in each of the preceding suggestions; they may also act independently, either by concen- 
trating lime in their structures or by precipitating lime from solution in taking molecules 
of carbonic acid from the bicarbonate. In either case local concentrations may result. 
Given lenses in which carbonate of lime is concentrated through any or all of these 
agencies or through others not here considered, a condition tending toward consolidation 
of the whole mass would bring about induration sooner in the local concentrations than 
in other parts of the mass. 
I have elsewheref described the crystallization of limestone in the Everglades of 
Florida, beneath the waters of shallow lagoons; and the geographic environment, in which 
the intraformational conglomerates of northern China and of other regions formed, may 
have been somewhat similar. We may conceive a shallow sea receiving fine terrigenous 
sediment, which accumulates so slowly as to allow opportunity for the chemical and 
physical reactions essential to local induration of more calcareous layers or lenses. ‘The 
flats of a great river delta, in which conditions of limestone formation are combined with 
conditions of intermittent submergence, exposure, and silting, are appropriate. In con- 
sequence of tides and storms the muddy bottom may now be an area of deposition, now 
the scene of agitation. Thus the limestone sheets or lenses may yield pebbles to a prac- 
tically contemporaneous deposit, and become bedded in sediment essentially like that in 
which they originated. 
Criteria by which to discriminate between the several processes of consolidation sug- 
gested are not readily found. Chemical segregation, like that commonly assumed as 
the method of concretionary growth, is apparently set aside by the absence of concentric 
structures; yet this is not conclusive. Concretions do not always show that structure. 
Mr. J. M. Arms Sheldonf figures a concretion, which, when sawed in two vertically and 
polished, showed distinct lines of stratification. ‘‘But,’’ he states, ‘‘with this exception, 
the mass looked perfectly homogeneous. There was not the slightest evidence of a nucleus 
or of concentric structure; the latter, however, developed gradually after long exposure 
to the air.’’ Several other concretions figured by Mr. Sheldon exhibit stratification, 
which extends from the clay through the concretion continuously. The stratification 
of the pebbles in the conglomerate, therefore, does not remove them from the class of 
* Warington: Journal, Chemical Society of London, vol. xx1, 1886. 
+ Conditions of sedimentation: Journal of Geology, vol. 1, p. 512. 
t Concretions from the Champlain Clays of the Connecticut Valley, by J. M. Arms Sheldon, Boston, 
1900. A memoir of thirty-eight pages on a collection of about 1,400 concretions, with 160 illustrations, 
privately printed by the University Press, Cambridge, Mass. 
