424 | S tructural and Ph ysiological Botany. 
often remaining, while the accompanying | beds of can 
receive their impressions, and preserve a correct image of — 
them. It frequently happens, however, that entire stems 
have become fossilised (Fig. 534, p. 418), and still show their 
cellular structure in a wonderfully beautiful manner. We 
have no knowledge of the length of time during which the 
Carboniferous period lasted ; but if it may be estimated from 
the thickness of the deposits of coal, many authorities would 
place it at millions of years. 
It is impossible to suppose that deposits of coal occurred only in. 
the Carboniferous period ; they are found in almost all strata higher 
in the series, but in a different from. In the uppermost we find Zeaz, the 
coal-formation of the present ; below it is /zgzzfe or brown coal, bear- 
ing sometimes a greater resemblance to peat, sometimes to the true coal 
which lies beneath it. The origin of the latter from peat is therefore © 
not improbable, and in some cases is unquestionable. The mode of 
formation of peat must therefore be first investigated. When Algze and 
other plants grow undisturbed in perfectly stagnant water, to the bottom. 
of which they fall quietly as they decay, the layer of organic matter 
thus formed grows gradually thicker and thicker ; the reeds, rushes, 
sedges, and Equiseta retreat from the shore, the dense matting of their 
roots gradually advancing farther and farther on the muddy soil. 
Every year the dead leaves and stems of the water-lilies, water-Ranun- 
culi, Potamogeta, and Lemnez fall to the bottom ; the mass of vegetable 
remains and the roots and rhizomes which penetrate among them be- 
come denser and denser, and more and more space is constantly taken 
from the water, until at length a densely interwoven substance results, 
resting ona soft layer of mud. Sphagna, Eriophora, marsh trefoil, 
Vaccinia, and other plants, establish themselves on this unstable 
foundation. The vegetable remains covered in this way, and protected 
against the action of the air, still decay, but not so completely ; they 
are continually losing oxygen, hydrogen, and nitrogen, while the 
carbon accumulates. The mass is also increasing upwards ; for long 
after the lower parts of the bog-mosses (Sphagna) which take an es- 
pecially important part in these processes have died off, the upper 
parts still maintain a vigorous life, drawing up the water from below 
like a sponge. The substructure, which was at first very soft, becomes — 
therefore gradually denser in the course of time, until at last a firm fead- 
moss is formed beneath the superficial vegetation. The ground is how- 
ever constantly kept moist by the peat, and a luxuriant vegetation is 
