Chap. VII.] TABLE OF SILURIAN FORMATIONS. 
143 
section as is seen in No. 1 (and which is everywhere apparent along the 
Silurian frontier) with the section from the Longmynd to the N.W. 
(No. 7), in which the lower divisions are fully developed, the geologist has 
before him, in Shropshire only, the whole Silurian system from its base to 
its summit, with the exception of the hiatus occasioned by the absence of 
a portion of the Llandovery Bocks explained at p. 89. In this same 
limited tract he also sees the most copious development of the Cambrian 
rocks known in Great Britain (the Longmynd), and finds, in short, nearly 
all the strata which by great undulations are spread over the Principality 
of Wales and various English counties ! 
A complete acquaintance with the numerous great dislocations which 
occur throughout the Silurian rocks can only be obtained by consulting 
the maps and sections of the Geological Survey. Reverting for a moment 
to such phenomena, the reader has only to refer to the section at p. 89 to 
see that, by the compound fractures of the crust in even the typical Silu- 
rian tract of Shropshire, the very oldest known sediments of the region are 
brought into contact with various overlying rocks. In that diagram, re- 
duced from the Survey Sections, we see, indeed, how different Upper 
Silurian rocks are broken up and forced in between an upper member of 
the Lower Silurian and the deep-seated Cambrian rocks of the Longmynd. 
And yet the geologist has only to follow the undulations of the Lower 
Silurian strata into Montgomeryshire to find the regular consecutive order 
reestablished between those rocks and the overlying strata; whilst in 
parts of Shropshire, Herefordshire, and South Wales the Upper Silurian 
rocks equally follow a clear ascending order. 
In terminating this sketch of the remarkable British Silurian region ex- 
hibited in the Map, and leaving the further description of the fossils for 
separate chapters, the following vignette is introduced, to represent at a 
Hook Point. 
Silurian Rocks op Marloes Bay, dipping under the Old Red Sandstone of Hook 
Point. Pembrokeshire. (From Sil. Syst. p. 392.) 
glance the only section on the sea-coast where a large proportion of the 
