122 
SILTJKIA. 
[Chap. VI. 
uniting the Wenlock and Ludlow in the Upper Silurian group, and is 
almost equally connected with both by structure and fossil contents. 
Since the preceding paragraph was published in the last edition of 
' Siluria,' Mr. C. Ketley has shown, in a paper read before the British 
Association in 1865, that so many fossils which characterize the Wenlock 
limestone occur in this superjacent shale, that it ought to be classed with 
the "Wenlock formation and not termed Lower Ludlow rock. This arrange- 
ment would agree with my own view, so far as regards the tract q£ 
Dudley, or wherever the cover of shale upon the limestone has no great 
thickness ; but it would be quite inapplicable to many parts of the Silurian 
region, where the vast thickness of shale which supports the Ludlow or 
Aymestry limestone must continue to be classed, for reasons to be assigned, 
with the superior formation as Lower Ludlow rock. 
A further acquaintance with the fossils of the Wenlock formation must 
be obtained by consulting the 10th Chapter, as also the descriptive mono- 
graphs mentioned in the Preface. The main object of the present work is 
not to direct attention to rare forms, but chiefly to those which are charac- 
teristic of the rocks. 
