462 - DYNAMICAL GEOLOGY. —  [Boox IIL. | 
precipitate the carbonate as an inorganic incrustation outside their 

own substance. Some observers have even maintained that this is 
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Fic. 173.—DIATOM-00ZE DREDGED UP BY THE Challenger EXPEDITION FROM A DEPTH 
or 1950 FatTuoms IN THE ANTARCTIC OcEAN. Lar. 53° 85 §.; Lona. 108° 38 E. 
MaeniFieD 300 DIAMETERS. 
the normal mode of production of calc-sinter, in large masses 
like those of Tivoli. It is certainly remarkable that this substance 
may be observed encrusting fibrous bunches of moss (Hypnum, &c.) 
when it can be found in no other part of the water-course, and this, 
too, ata spring containing only 0:°034 of carbonate. It is evident 
that the deposit of calc-sinter cannot be due to mere evaporation, 
otherwise it would be more or less equally spread along the edges 
and shallow parts of the channel. It arises first, from the decom- 
position of dissolved carbonic acid by the living plants, and it proceeds 
along their growing stems and fibres. Subsequently evaporation and 
loss of carbon dioxide cause the carbonate to be precipitated over 
and through the fibrous sinter till the substance may become a solid 
crystalline stone. Varieties of sinter are traceable to original dif- 
ferences in the plants precipitating it. Thus at Weissenbrunen, near 
Schalkau, in central Germany, a cavernous but compact sinter is 
made by Hypnum molluscum, while a loose porous kind gathers upon 
Didymodon capillaceus.* 
Some marine algz, as above noticed, abstract calcium carbonate 
from sea-water and build it up into their own substance. A nulli- 
pore (Lithothamniwm nodosum) has been found to contain about 84 
= cent. of calcium carbonate, 54 of magnesium carbonate, with a 
ittle phosphoric acid, alumina, and oxides of iron and manganese.? 
Considerable accumulations of such calcareous algee take place along 
some shore lines. Broken up by the waves and thrown ashore with 
fragmentary shells or other organisms, the calcareous detritus is 
cemented into solid stone by the solvent action of the carbonic acid 
of rain or oceanic water. 
In the formation of extensive beds of bog iron-ore the agency of 
1 See V. Schauroth, Z. Deutsch. Geol. Ges. iii. (1851), p. 137. Cohn, Neues Jahrb. 
1864, p. 580, gives some interesting information as to the plants by which the sinter is 
formed, and their work. In Scotland Hypnwm commutatum is a leading sinter-former. 
2 Giimbel, Abhandl. Bayerisch. Akad. Wissensch. xi. 1871. 
