WENLOCK FORMATION. 
465 
Fi<?. 536. 
gradual has been our progress in tracing back the signs of 
vertebrata to formations of high antiquity. Such facts may 
be useful in warning us not to assume too hastily that the 
point which our retrospect may have reached at the present 
moment can be regarded as fixing the date of the first intro¬ 
duction of any one class of beings upon the earth. 
2 . Wenlock Formation. —We next come to the Wenlock 
formation, which has been divided (see Table, p. 458) into 
Wenlock limestone, Wenlock shale, and Woolhope limestone 
and Denbighshire grits. 
a, Wenlock Limestone, — This limestone, otherwise well 
known to collectors by the name of the Dudley Limestone, 
forms a continuous ridge in Shropshire, ranging for about 20 
miles from S.W. to N.E., about a mile distant from the nearly 
parallel escarpment of the Aymestry limestone. This ridgy 
prominence is due to the solidity of the rock, and to the soft¬ 
ness of the shales above and below it. Near Wenlock it 
consists of thick masses of gray subcrystalline limestone, re¬ 
plete with corals, encrinites, and trilobites. It is essentially 
of a concretionary nature ; and the con¬ 
cretions, termed ‘‘ball-stones” in Shrop¬ 
shire, are often enormous, even 80 feet 
in diameter. They are of pure carbon¬ 
ate of lime, the surrounding rock being 
Sometimes 
in the Malvern Hills 
this limestone, accord¬ 
ing to Professor Phil¬ 
lips, is oolitic. 
Among the corals, 
in which this forma¬ 
tion is so rich, 53 spe¬ 
cies being known, the 
“ chain - coral,” Haly- 
sites catenularius (Fig. 
536), may be pointed 
out as one very easily recognized, and wide¬ 
ly spread in Europe, ranging through all 
parts of the Silurian group, from the Ayme¬ 
stry limestone to near the bottom of the 
Llandeilo rocks. Another coral, the Favo- 
fn (Fig. 537), is also met with 
in profusion in large hemispherical masses, 
which break up into columnar and prismatic fragments, like 
that here figured (Fig. 537, d). Another common form in the 
^ Murchison’s Siluria, chap. vi. 
20 * 
more or less argillaceous.* 
Fig. 53T. 
Halysites catenularius, Liuu. 
sp. Upper and Lower Si¬ 
lurian. 
Favosites Gothlandica, 
Lam. Dudley. 
a. Portion of a large 
mass; less than the 
natural size. 6. 
Magnified portion, 
to show the pores 
