344 Subdivisions of the Palzozoic Strata of Great Britain. 
of the older organic types disappear, and new types take their 
place. ‘The sections are usually broken and discontinuous ; and 
language in common use), we have the commencement of a hew 
system. Without counting the vast thickness of the Longmynd 
slates, the thickness of the Cambrian series, where well devel- 
oped, is, I think, more than 25,000 feet. 
Lower Paleozoic Division continued. 
(a. May Hill sandstone; Pentamerus (or Norbury) lime- 
stone, 
2 1. Wenlock group 4 Nay! “pe Lower Wenlock) limestone. 
o Wenlock 
ip Upper re west) — limestone. 
2 a. Lower Ludlo 
a <j Aymestry relietact 
n 
d. Tilesto 
The aggregate thickness of the series about 5000* feet. 
° Ludlow group c. Upper r La diced sek. 
The introduction of the May Hill sandstone as a part of the 
Wenlock group is the only important change I have made in the 
corresponding portion of the “ Tabular View” which is ee refix ed 
such ay in nace as a “Middle Silurian Group,” which in- 
separably links together the Cambrian and Silurian series, and 
makes them into one system. The May Hill subgroup is not 
unfrequently discordant to the older groups, on which it rests ; 
and its fossils unite it unequivocally to the Wenlock group. It 
_ must therefore, both on physical and paleontological evidence, 
cut off from the shelly sandstone of Caer Caradoc and Hor- 
derly ; to which it is discordant in position, and with which its 
palzontological relations are not compar atively so near as to t the 
enlock group. It is in fact, as now given in the Table, an 
integral partion of the Wenlock group. 
* T obtain this nu nike from an estimate of the thickness of the “ Upper Silurian” 
rocks, by Dr. My —— to the thickness of the May Hill sandstone, as given by 
Professor Phil lips. Aggregates of this kind are frequently too great. For, a5 & 
cg: tule, where one of a set of connected groups rises ah 
another group will ever descend below it. For example. The sent pe 
tunes of Presteign is rock, but the Wenlock limestone is quite degene- 
rate. At Wenlock thé tikopacone forms a a grand terrace, but the Aymestry limestone 
has almost vanished. At ic kiein cg the poh corel limestone is a grand rock, 
the Wenlock limestone is d e. Many other examples might be 
