EXPLANATION OF THE PLATES AND WOODCUTS. 



sent the relative position of the trees with respect to the horizontal level, but not the pro- 

 portional distance of the different trees. 

 Fig. 6. Section of the strata overlying the trees. 



PLATES XVIIL and XIX. 



Illustrate Mr. Bowerbank's paper on the SiUceous Bodies of the Chalk, Greensands and 

 Oolites : p. 181 to 194. 



Plate XVIIL 

 Chalk Flints. 



Fig. 1 . A view of part of a section of a nodular chalk flint from Gravesend, Kent, exhibiting 



the spongeous tissue in its most perfect form, seen as an opaque object with a leiberkuhn 



and a power of 120 linear. 

 Fig. 2. Represents the minute tubular sponge-fibres on the surface of a flint-cast of a Galerite, 



at the depression opposite the marginal orifice of the base of the shell, seen as an opaque 



object by direct light with a power of 120 linear. 

 Fig. 3. Spicula on the interior surface of a Wiltshire chalk flint, seen as an opaque object by 



direct hght with a power of 35 linear. 

 Fig. 4. Some of the largest of the spicula from the interior of a chalk-flint from Wiltshire, 



which contained several species of sponges and corals, seen as opaque objects by direct 



light with a power of 35 linear. 



Plate XIX. 

 Greensand Cherts. 



Fig. 1 . Sponge-fibres and spicula in the Upper Greensand Chert from Fovant, Wilts, viewed 



as an opaque object with a leiberkuhn and power of 50 linear. 

 Fig. 2. Sponge-fibres in the Greensand Chert from Lyme Regis, Dorsetshire, viewed as an 



opaque object with a leiberkuhn and power of 50 linear. 

 Fig. 3. A fibre in the specimen from which fig. 3 was drawn, seen as a transparent object with 



a power of 50 linear, exhibiting the flocculent appearance of the surface of the tissue when 



viewed by transmitted hght. 



WOODCUT. 



Section to illustrate Mr. Bowman's notes on a patch of Silurian rocks on the northern coast 

 of Denbighshire : p. 195. 



