366 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 2 



covered with sand, the wood and other tissues gradually decaying 

 and crumbling into dust wdiich was ultimately swept away by the 

 fiowdng water; a mold or cavity was left where the remains of the 

 tree had been. Subsequently more sand filled the space and thus 

 casts of the tree bases and roots were made. Large casts of Lepido- 

 dendron stems illustrating this method of preservation were dis- 

 covered many years ago in rocks of Carboniferous age near Glasgow 

 in Scotland, and are now preserved as a natural monument. 



One of the aims of geologists is to determine the relative ages of 

 rocks by means of the order of superposition and the nature of the 

 animal and plant fossils. It has been possible to arrange the rocks 

 of the earth's crust in the order of their geologic age and thus to 

 furnish what may be called a table of contents of the history of 

 the world. The history of the earth, like the history of peoples, is 

 conveniently divided into periods or chapters, and in recent years 

 it has been possible to give estimates of the actual age in years of 

 the several eras and periods represented by the sedimentary and 

 igneous rocks. To the oldest known set of rocks the comprehensive 

 term " pre-Cambrian " has been given ; of the life of that era, which 

 in duration probably equaled or even exceeded all the other eras 

 and periods put together, we know practically nothing. It must 

 have been in the course of this pre-Cambrian age that a lifeless world 

 became the scene of the first act in the drama of life. Life prob- 

 ably began in the sea, but we can never expect to discover in the 

 rocks traces of the inconceivably minute bodies of the first or the 

 most primitive representatives of the organic world. From the rocks 

 of the succeeding Cambrian, Ordovician, and Silurian periods sev- 

 eral fossil seaweeds have been obtained; also, from Silurian rocks, 

 a very few imperfectly preserved remains of terrestrial plants. 



It is not until we pass to the sediments of the Devonian period 

 that satisfactory records of land plants are available. Many years 

 ago the late Sir William Dawson, of Montreal, described numerous 

 Devonian plants from rocks of the Gaspe Peninsula, and much 

 more recently there was discovered in Aberdeenshire, Scotland, at 

 Rhynie, a bed of flinty rock, or chert, full of beautifully preserved 

 petrified stems and spore cases of small plants which have added 

 greatly to our knowledge of one of the most ancient floras in the 

 world. The Rhynie chert may be described as a petrified sample 

 of a peaty swamp. We can reconstruct a scene in Devonian Scot- 

 land : Pre-Cambrian mountains overlooking a flat expanse of 

 swampy ground covered with a green carpet of plants a few inches 

 high; not far away, we may suppose, were fumaroles providing 

 heated water which dissolved silica from the rocks and subsequently 

 deposited it in the tissues of the peat-forming plants. One of the 



