62 LINKS WITH THE PAST [ch. iv 



primeval forest each had grown, what chance had 

 originally cast them on the waters, and piloted 

 them to this desert shore 'os). A photograph re- 

 produced in Amundsen's book on The North West 

 Passage shows the beach on the Alaskan coast 

 strewn with drifted timber (39). For the accompanying 

 photograph (Fig. 7) of the flood-plain of the Colorado 

 River (40), I am indebted to Professor MacDougal of 

 the Desert Research Laboratory at Tucson, Arizona, 

 who in a recent letter writes, ' During times of high- 

 water a thin sheet of flood covers the flat for many 

 miles and bears drift-wood so thickly that it is 

 difficult to push a boat through it.' The drift-wood 

 consists of poplar, willow, pine, and juniper, ' the last 

 two have been brought from the upper river, from 

 as far away as a thousand miles.' A picture such as 

 this affords an admirable example of the wealth 

 of material available for preservation in a fossil state. 

 It is only in the minority of cases that the 

 accidents of preservation of fragments of ancient 

 floras have given us the means of investigating the 

 internal structure of the plant organs. It is far more 

 frequently the case that fossil plants are represented 

 only by a carbonised film on the surface of a piece of 

 shale or other rock : the actual substance of the 

 plant has been converted into a thin layer of coal, 

 and though the venation and other surface-features 

 may be clearly revealed, the internal tissues have 



