GEOGNOSY AND GEOLOGY. 195 
of deer, oxen, &c., with their tracks, are met with; as also the products of 
human industry. 
v. Silicious Tufa, a deposit from hot silicious springs. It forms either 
conical hills on whose summit the spring is generally revealed, or else the 
filling up of cavities, as in the crater of the great Geyser of Iceland 
(pl. 44, fig. 17). 
c. Soda and Salt, which are sometimes deposited on the edges of lakes. 
d. Deposits of Borax (boracic acid). 
e. Deposits of Alum and Magnesia. 
f. Beds of oxyde of Iron, on the bottom of lakes. 
C. Masses which have arisen directly from the Decomposition or Destruction 
of Rocks. 
a. Piles of loose blocks, occurring on the sides of mountains, and 
sometimes covered with soil, in which case they may give rise to the 
phenomena of land slides. 
b. Gravel Beds, produced by the weathering of rocks. Thus we have 
granite, gravel, syenite-gravel, &c. These gravels are sometimes cemented 
anew, and produce the so-called regenerated granite or syenite, and granitic 
conglomerate. 
c. Earthy Masses produced from the subjacent rocks, and occupying 
their original position. 
d. Masses produced by the Decomposition of Plants, among which 
peat stands pre-eminent. We distinguish wood, leaf, and moss peat, 
according as one or other of these substances contributed principally to the 
formation of the peat bog. The deposits generally occur in depressions, yet 
sometimes in elevated places. Their origin presupposes a water-tight soil, 
such as is produced in particular by clay and granitic gravel. The beds 
vary in extent and thickness, the latter being greater in the middle than on 
the borders, as wowld naturally result from a deposit in a trough or basin. 
Peat often includes mineral bodies, among which may be mentioned 
pyrites, gypsum, yellow and brown iron-stone, limonite, phosphate of iron, 
and retinasphaltum. The distinction is made into green and black moss, 
according as the moor is overgrown with vegetation or not; another 
distinction may be made into peat from marine and trom fresh water 
plants. 
e. Masses produced by Animal Agencies. Here belong the beds of 
silicious meal, which are really aggregations of infusorial shells, mixed with 
the pollen of pines, &c. Here also are to be ranged those deposits of 
guano occurring on the coast of Patagonia, Peru, &c. 
ICONOGRAPHIC ENCYCLOPAZDIA.— VOL. I. 40 625 ~ 
