468 
ELEMENTS OF GEOLOGY. 
occurs. It contains the usual Wenlock fossils, but with the 
addition of some common in the uppermost Ludlow rock, 
such as Chonetes lata and Bellerophon trilohatm. The chief 
fossils of the Woolhope limestone are lllmnus JBarriensis^ 
Homalonotus delphinocephalus (Fig. 544), StropJiomena im¬ 
brex^ and Bhynchonella Wilsoni (Fig. 531). The latter at¬ 
tains in the Woolhope beds an unusual size for the species, 
the specimens being sometimes twice as large as those found 
in the Wenlock limestone. 
In some places below the Wenlock formation there are 
shales of a pale or purple color, which near Tarannon attain a 
thickness of about 1000 feet; they can be traced through 
Radnor and Montgomery to North Wales, according to 
Messrs. Jukes and Aveline. By the latter geologist they 
have been identified with certain shales above the May-Hill 
Sandstone, near Llandovery, but, owing to the extreme scarci¬ 
ty of fossils, their exact position remains doubtful. 
3. Llandovery Group—Beds of Passage. —We now come to 
beds respecting the classification of which there has been^ 
much difference of opinion, and which in fact must be con¬ 
sidered as beds of passage between Upper and Lower Siluri¬ 
an. I formerly adopted the plan of those who class them as 
Middle Silurian, but they are scarcely entitled to this distinc¬ 
tion, since after about 1400 Silurian species have been com¬ 
pared the number peculiar to the group in question only 
gives them an importance equal to such minor subdivisions 
as the Ludlow or Bala groups. I therefore prefer to regard 
them as the base of the Upper Silurian, to which group they 
are linked by more than twice as many species as to the 
Lower Silurian. By this arrangement the line of demarka- 
tion between the two great divisions, though confessedly ar¬ 
bitrary, is less so than by any other. They are called Llando¬ 
very Rocks, from a town in South Wales, in the neighbor¬ 
hood of which they are well developed, and where, especial¬ 
ly at a hill called Noeth Grtig, in spite of several faults, their 
relations to one another can be clearly seen. 
a. Upper Llandovery or May-Hill Sandstone. —The May- 
Hill group, which has also been named “ Upper Llandovery,” 
by Sir R. Murchison, ranges from the west of the Longmynd 
to Builth, Llandovery, and Llandeilo, and to the sea in Mar¬ 
low’s Bay, where it is seen in the cliffs. It consists of brown¬ 
ish and yellow sandstones with calcareous nodules, having 
sometimes a conglomerate at the base derived from the 
waste of the Lower Silurian rocks. These May-Hill beds 
were formerly supposed to be part of the Caradoc formation, 
but their true position was determined by Professor Sedg- 
