24 BIOLOGICAL LECTURES. 



only represented by their mycelia, but, as shown by Renault 

 and others, even the bacteria are now regarded as well-known 

 forms among carboniferous plants. 



Plants which are enclosed in a deposit of compact clay while 

 yet living, or before any essential decay has taken place, often 

 exhibit the most perfect preservation, since they have been 

 hermetically sealed, and thus have retained all their structural 

 features in an unaltered condition for an unknown number of 

 thousands of years. Notable examples of this occur in the 

 various interglacial deposits. In the Leda clays at Montreal, 

 when fresh cuttings are being made, one may see leaves of the 

 common Vallisneria spiralis as perfect in all their structural 

 features as if freshly gathered from the adjacent river. But 

 the delicate structure has been so far influenced by its long 

 enclosure in the moist clay that desiccation leaves nothing but 

 a mere impression of the former organ. In the same formation 

 near Ottawa, fragments of perfectly preserved wood are enclosed 

 in nodules of clay, while at Toronto, from the same formation 

 again, we obtain the Osage orange with all its structure perfectly 

 preserved, and red cedar, which is not only perfect as to struc- 

 ture, but which possesses the characteristic shreddy bark, red 

 color, and distinctive odor. Such woods are sectioned in the 

 microtome precisely as if taken from a living plant. 



While woods from the Pleistocene are often preserved as just 

 described, it more commonly happens in older formations that 

 the mass has been wholly or in part carbonized, or that it has 

 become so charged with mineral matter that it has passed into 

 a petrified state. These two forms of alteration commonly 

 accompany one another. 



(a) Carbonization. Plant remains which are covered shortly 

 after decay arises, so that the latter continues under exclusion 

 of air, become converted into a mass of carbon which, accord- 

 ing to circumstances, retains the original structural features 

 more or less perfectly. This carbonization results from with- 

 drawal of the elements of water under the peculiar conditions 

 established, and if pressure and heat are subsequently brought 

 into operation, there results a compact coal, and possibly cer- 

 tain oily and gaseous products, as may be noted in the case 



