XX. Explanation of a Supplementary Plate to the Paper on Vegetable 

 Remains preserved in Chalcedony, printed in the Second Volume 

 of the Transactions of the Geological Society. 



By J. Mac Culloch, m.d. f.l.s. President of the Geological Society, 

 Chemist to the Ordnance, Lecturer on Chemistry at the Royal 

 Military Academy, and Geologist to the Trigonometrical Survey. 



[Read 19th May, 1815.] 



Fig. 1.* jLHIS figure is given for the purpose of explaining 

 the mode in which the fibrous disposition of chlorite is effected. 

 The crystallized arrangement which is here so visible is generally 

 observed in those chlorite fibres which occur in transparent quartz. 

 I have never observed it in chalcedony, yet there is little doubt that 

 a similar disposition is equally the cause of the fibrous appearance 

 so common in the specimens which have been figured in the for- 

 mer plates, the chief differences consisting in the smaller size and 

 less perfect crystallization of the chlorite scales. 



Fig. 2. The crystals of chlorite magnified. 



Fig. 3. Although in the former plates I figured some specimens 

 which I imagined to be confervse, I had not then procured any 

 example in which that regularity of structure, which is so conspicu- 

 ous in some of the plants of this family, could be observed. The 

 present figure exhibits that regularity in a striking manner. 



Fig. 4. This figure affords a second example of that regular ra- 

 mification which botanists will immediately recognize as character- 

 izing many of the plants of this tribe ; although for the reasons 

 already assigned in the paper, it will be fruitless to attempt to dis- 

 cover the species. 



* PI. 27. 



