66 



We may, as a homely illustration, compare such an impression 

 and cast with a mould in which a jelly has been allowed to set. 

 The mould corresponds to the impression, and is the reverse of the 

 original, while the jelly is comparable to a cast. When the mould 

 was manufactured it was fashioned round some hard substance 

 which corresponds to the stem of our plant. An empty mould 

 represents an impression, and the space it encloses, the empty space 

 left when the plant has entirely decayed away. The jelly represents 

 the cast. Most of the photographs reproduced here are from casts 

 or impressions. 



Petrifactions. — Casts and impressions are by far the most 

 common type of preservation among fossil plants of all geological 

 ages. They have this peculiarity that, if the surface had not 

 begun to decay before the impression vi-^as taken, they show the 

 external features. But they never have the internal structure 

 preserved. Often, indeed, especially among stems oi Lepidodendron 

 and Sigillaria^ the stems had partially decayed before the impres- 

 sion was made, and thus not the true external surface, but a 

 partially decayed (decorticated) condition is exhibited by such 

 specimens. 



In the case of the second type of preservation, the petrifaction, 

 what has happened is altogether different. Let us imagine the stem 

 of a fallen tree permeated and soaked with v^ater highly charged 

 with lime (calcium carbonate) or quartz (silica). As the water perco- 

 lates through the tree, and the tissues decay cell by cell, the calcium 

 carbonate tends to crystallise out. Each cell as it disappears is 

 replaced by an exactly similar mass of crystaline calcium carbonate 

 or silica, which resembles the original cell in size, shape, and 

 peculiarities — in fact, in all respects. This is what has happened in 

 the case of a petrifaction. A tissue of living cells has been replaced 

 by a precisely similar mass of mineral units. Thus the internal 

 structure is preserved to us, and if we cut thin sections, by carefully 

 grinding down a thin slice of a petrifaction until it is transparent, 

 we can examine them under the microscope, and in many cases they 

 will reveal the internal structure almost as perfectly as a section of 

 a living plant cut by means of a razor. Plates on Pages ii, 12, 26, 

 30, 51, 60, and 63 are photographs of sections of petrifactions. 



Unfortunately, as a rule, petrifactions are of very restricted 

 occurrence. In the British Upper Carboniferous rocks they are only 

 found embedded in calcareous nodules, known as "coal-balls," in 

 one or two seams in the South Lancashire and Yorkshire coalfields. 



It may be wondered how it is ascertained that impressions of a 

 certain root and a certain stem belonged originally to plants of the 

 same species, which are found somewhere else preserved as petrifac- 

 tions. This is the task of the Palaeobotanist, and the labours of the 

 last forty years in this direction have made it possible to restore 

 many of the more important Carboniferous plants. This is effected 

 by the correlation in structure of two different organs, or by the 

 discovery of exceptional specimens in which they are still united, or 

 which show both the external features and the internal structure. 



The reader, bearing in mind the extreme antiquity of the Car- 

 boniferous period, will not, perhaps, be surprised to hear that the 

 flora at that time differed very greatly from the flora of to-day. 



At the present day the Dicotyledons and Monocotyledons pre- 

 dominate over all the other groups of the higher plants. These are 

 the plants bearing flowers. 



