608 WALTER H. BUCHER 
Under the microscope these stromatoliths exhibit a structure of 
delicate layers identical with that of the odlites, with which they 
are often connected by all stages of transition’ They differ 
greatly, both in structure and in origin, from the coarse calcareous 
crusts which are formed by thick, felted masses of fresh-water 
algae and mosses on shells and pebbles.?, The two can easily be 
distinguished when found associated in the same formation, as, 
for instance, in the Miocene limestones of the Rhine valley.’ 
The stromatoliths are the sedimentary equivalent of the cal- 
careous and siliceous “‘sinter” of the hot springs; stromato- 
lithic crusts of limonite are commonly found in lake and bog 
ores. 
In the simple experiments with iron chloride, described in the 
first part of this paper, similar growths were observed in the same 
cases in which spherites with odlitic structure were produced. . In 
these experiments flat expansions formed below the massive crust 
which sealed the liquid, and, to a smaller degree, also at the bottom 
of the vessel, corresponding in thickness to the radius of the odlitic 
grains and showing the same delicate concentric structure. There 
can be little doubt but that they represent the experimental repro- 
duction of stromatoliths. 
Subsequent alteration of odlites.—Another, more fundamental, 
source of error in the interpretation of the origin of odlites has been 
the more or less altered condition of most fossil odlites. Many cal- 
careous odlites, for instance, have suffered complete recrystalliza- 
tion which, starting usually in the center of the grains,* replaces 
their original structure by a few crystals, or even a single 
large calcite individual, concentrating all impurities in a thin 
layer at their periphery. Such extreme cases have been inter- 
1 Cf., e.g., Bucher, Geogn. Jahresh., XXVI (1913), 78-79, and Pl. I, Fig. 22. 
2Cf., e.g., J. M. Clarke, ‘“‘The Water Biscuit of Squaw Island, Canandaigua 
Lake, N.Y.,” Bull. N.Y. State Mus., VIII (1900), No. 39, 195; W. Schmidle, “‘Post- 
glaciale Ablagerungen im nordwestl. Bodenseegebiet,”’ N. Jahrb. fiir Min., etc. (1910), 
Part II, to5—22. 
3 Bucher, op. cit., pp. 80-81. 
4 Cf. the illustrations and the discussion of the mechanism of this process in Reis, 
Geogn. Jahresh., XXII (1909), 227-31, and Pl. 11, Figs. 25-30. 
