IV] PRESERVATION OF TISSUES. 83 



structural features. This minute cone-in-cone structure is not 

 uncommon in petrified tissues ; it is precisely similar in appear- 

 ance to that described by Cole' in certain minerals. The 

 crystallisation has been set up along lines radiating from 

 different centres, and the particles of the tissue have been 

 pushed as it were along these lines. 



A somewhat different crystallisation phenomenon is illus- 

 trated by the extremely fine section of a Lepidodendroid plant 

 shown in fig. 1.5. The tissues of the primary and secondary 

 wood {x^ and x^) are well preserved throughout in silica, but 

 scattered through the siliceous matrix there occur numerous 

 circular patches, as seen in the figure. One of these is more 

 clearly shown in fig. 14 B drawn from a longitudinal section 

 through the secondary wood, x' ; it will be noticed that where 

 the concentric lines of the circular patch occur, the scalariform 

 thickenings of the tracheids are sharply defined, but imme- 

 diately a tracheid is free of the patch these details are lost. It 

 would appear that in this case silicification was first completed 

 round definite isolated centres, and the secondary crystallisation 

 in the matrix partially obliterated some of the more delicate 

 structural features. The same phenomenon has been observed 

 in oolitic rocks'', in which the oolitic grains have resisted 

 secondary crystallisation and so retained their original structure. 



Among the most important examples of silicified plants are 

 those from a few localities in Central France. In the neighbour- 

 hood of Autun there used to be found in abundance loose nodules 

 of siliceous rock containing numerous fragments of seeds, twigs, 

 and leaves of different plants. The rock of which the broken 

 portions are found on the surface of the ground was formed 

 about the close of the Carboniferous period. 



At the hands of French investigators the microscopic 

 examination of these fragments of a Palaeozoic vegetation have 

 thrown a flood of light on the anatomical structure of many 

 extinct types. Sometimes the silica has penetrated the cavities 

 of the cells and vessels, and the walls have decayed without 

 their substance being replaced by mineral material. Sections 

 of tissues preserved in this manner, if soaked in a coloured 



1 Cole (94), figs. 1 and 3. ' Barker (95) p. 233, fig. 56. 



6—2 



