104 
S1LUKIA. 
[Chap. V. 
Chonetes lata, Murchisonia Lloydii, Bellerophon trilobatus, B. carinatus, 
Orthoceras annulatum, &c. 
In the general Table of Colours attached to the Map, these ' Denbigh 
grits ' are therefore placed at the bottom of the Wenlock formation. But 
as, with the exception of parts of North Wales, where the beds are sandy, 
the Wenlock formation is eminently distinguished by its purely argilla- 
ceous base, these exceptional examples have been first disposed of, and we 
now proceed to consider the Upper Silurian rocks from their base upwards, 
as developed in the original Silurian Region. 
