
Parr II. Sror. iii. § %] VEGETABLE FORMATIONS. 461 
lakes that preceded the mosses), and canoes hollowed out of single 
trees." 
3. Mangrove Swamps.—On the low moist shores and river 
mouths of tropical countries, the mangrove tree plays an important 
geological part. It grows in such situations in a dense jungle, some- 
times twenty miles broad, which fringes the coast as a green selvage, 
and runs up, if it does not quite occupy, creeks and inlets. The man- 
grove flourishes in sea-water even down to low-water mark, forming 
there a dense thicket, which, as the trees drop their radicles and take 
root, grows outward into the sea. It is singular to find terrestrial 
birds nestling in the branches above and crabs and barnacles living 
among the roots below. By this network of subaqueous radicles 
and roots the water is filtered of its sediment, which, retained among 
the vegetation, helps to turn the spongy jungle into a firm soil. On the 
coast of Florida the mangrove swamps stretch for long distances asa 
belt from five to twenty miles broad, which winds round the creeks 
and inlets. At Bermuda the mangroves co-operate with grasses and 
other plants to choke up the creeks and brackish lakes. In these waters 
calcareous algz abound, and, as their remains are thrown up amidst 
the sand and vegetation, they form a remarkably calcareous soil.? 
4. Diatom Harth or Ooze.—As the minute siliceous plants 
called diatoms occur both in fresh and salt water, the deposit formed 
from their congregated remains is found both on the sites of lakes 
and on the sea-floor. ‘“ Infusorial” earth and “tripoli powder” con- 
sist mainly of the frustules and fragmentary débris of diatoms which 
have accumulated on the bottoms of lacustrine areas. The purer 
varieties contain 90 to 97 per cent. of silica. They form beds some- 
times upwards of thirty feet thick. (Richmond in Virginia and 
Bilin in Bohemia.) Towards the Antarctic circle the Challenger met 
with Diatomacez in abundance, both in the surface waters of the 
ocean and on the bottom. They form at depths of from 1260 to 1975 
fathoms a pale straw-coloured deposit, which when dried is white 
and very light (Fig. 173). 
5. Chemical Deposits.—But, besides giving rise to new 
formations by the mere accumulation of their remains, plants do so 
also both directly and indirectly by originating or precipitating 
chemical solutions. The most conspicuous example of this action is 
the production of calc-sinter. Some plants (several species of Chara, 
for instance) have the power of decomposing the carbonic acid dis- 
solved in water, and precipitating calcium carbonate within their 
own cell walls. Others (such as the mosses Hypnum, Brywm, éc.*) 
1 On the composition, structure, and history of peat-mosses, consult Rennie’s 
“Essays on Peat-moss,” Edinburgh, 1810. Templeton, Trans. Geol. Soc. vy. p. 608. 
Pokorny, Verhand. Geol. Reichsanst. Vienna, 1860; Senft, “ Humus-, Marsch-, Torf- und 
Limonit-bildungen,” Leipzig, 1862; J. Geikie, Trans. Roy. Soc. Edin. xxiv. p. 363. 
For a full list of plants that supply vegetable material for the formation of peat, see 
T. Rupert Jones, Proc. Geologist’s Association, 1880, p. 217. 
2 See Nelson, Q. J. Geol. Soc. ix. p. 200, et seq. - 
- % Also phanerogams, as Ranunculus and Potamogeton. 
