(JGO MIDDLE SILURIAN ROCKS. [Ch. XXVII. 



abundant in this shale, and occurs more sparingly in " the Ludlow." 

 Of these fossils, which are more characteristic of the Lower' Silurian, 

 I shall again speak m the sequel (p. 565). 



b. Woolhope Limestone and Grit. — Though not always recognized 

 as a separate subdivision of the Wenlock, the Woolhope beds which 

 underlie the Wenlock shale are of great importance. Usually they 

 occur as massive or nodular limestones, underlaid by a fine shale or 

 flagstone ; and in other cases, as in the noted Denbighshire sandstones, 

 as a coarse grit of very great thickness. This grit forms mountain 

 ranges through North and South "Wales, and is generally marked by 

 the great sterility of the soil where it occurs. It contains the usual 

 Wenlock fossils, but with the addition of some common in the upper- 

 most Ludlow rock, such as Chonetes lata and Bellerophon trilobatus* 

 The chief fossils of the Woolhope limestone are Ulcenus Larriensis, 

 .Homalonotus delphinocephalus (fig. 639), Strophomena imbrex, and 

 Rhynchonella Wilsoni (fig. 626). The latter attains, in the Woolhope 

 beds an unusual size for the species, the specimens being sometimes 

 twice as large as those found in the Wenlock limestone. 



MIDDLE SILURIAN ROCKS. 



Upper Llandovery. — a. Tarannon shale. — Next below the Wenlock 

 formation are found in some places the Tarannon shales or pale slates, 

 sometimes purple, which are of small thickness near Llandovery, but 

 acquire large dimensions at Tarannon in Montgomeryshire, being, 

 according to Ramsay, about 1000 feet thick in that locality ; according 

 to Mr. Jukes and Mr. Aveline, they form a band of great persistence, 

 extending from Llandovery, through Radnor and Montgomerey, to 

 North Wales. Fossils are rare in them, and most of them are of spe- 

 cies common to the Wenlock formation. 



b. May-Hill Sandstone. — Next in descending order comes the May- 

 Hill sandstone, which may be advantageously studied at May-Hill in 

 Gloucestershire, and in the Malvern and Abberley Hills ; its position 

 was first accurately determined by Prof. Sedgwick, who considered it 

 as the true base of the Upper Silurian rocks. In the Malvern range it 

 attains a thickness of 600 feet. These beds were formerly called 

 Upper Caradoc, when they were supposed to be part of the Caradoc 

 formation, to be mentioned in the sequel; but this nomenclature has 

 been abandoned for good reasons, with which I need not here detain 

 the reader. They are named Upper Llandovery by Sir R. Murchison 

 in the last edition of his " Siluria." Conjointly with the Lower Llan- 

 dovery rocks, they have been called the Pentamerus beds, because Pen- 

 tamerus Icevis is very abundant in them, a brachiopod wanting both 

 in the Upper and Lower Silurian. It is usually accompanied by P. 



* Sedgwick, Quart. Geol. Journ., vol. i. p. 20, 1845. 



