IV] PLANTS AND COAL. 75 



see, then, that trees may have resisted decay for a siifBciently 

 long time to allow of a considerable deposition of sediment. It 

 is very difficult to make any computation of the rate of depo- 

 sition of a particular set of sedimentary strata, and, therefore, to 

 estimate the length of time during which the fossil stems must 

 have resisted decay. 



The protective qualities of humus acids, apart from the 

 almost complete absence of Bacteria^ from the waters of Moor- 

 or Peat-land, is a factor of great importance in the preservation 

 of plants against decay for many thousands of years. 



From examples of fossil stems or leaves in which the organic 

 material has been either wholly or in part replaced by coal, we 

 may pass by a gradual transition to a mass of opaque coal in 

 which no plant structure can be detected. It is by no means 

 uncommon to notice on the face of a piece of coal a distinct 

 impression of a plant stem, and in some cases the coal is 

 obviously made up of a number of flattened and compressed 

 branches or leaves of which the original tissues have been 

 thoroughly carbonised. A block of French coal, represented 

 in fig. 13, consists very largely of laminated bands composed 

 of the long parallel veined leaves of the genus Cordaites 

 and of the bark of Lepidodendron, Sigillaria, and other Coal- 

 Measure genera. The long rhizomes and roots below the coal 

 are preserved as casts in the underclay. 



In examining thin sections of coal, pieces of pitted tracheids 

 or crushed spores are frequently met with as fragments of 

 plant structures which have withstood decay more effectually 

 than the bulk of the vegetable debris from which the coal was 

 formed. 



The coaly layer on a fossil leaf is often found to be 

 without any trace of the plant tissues, but not infrequently 

 such carbonised leaves, if treated with certain reagents and 

 examined microscopically, are seen to retain the outlines of the 

 epidermal cells of the leaf surface. If a piece of the Carbon- 

 aceous film detached from a fossil leaf is left for some days in 

 a small quantity of nitric acid containing a crystal of chlorate of 

 potash, and, after washing with water, is transferred to ammonia, 

 1 Warming (96) p. 170. 



