228 Geological Society. 



occupy the western parts of Shropshire and Herefordshire, and are 

 prolonged from N.E. to S.W., through Radnorshire, Brecknock- 

 shire, and Caermarthenshire, with descriptions of the accompanying 

 rocks of intrusive or igneous characters," by Roderick Impey Mur- 

 chison, Esq. F.G.S. F.R.S. &c, was read. 



In this part of the memoir the author separates into distinct for- 

 mations by the evidences of fossils and the order of superposition, 

 the upper portions of those vast sedimentary accumulations, which 

 have hitherto been known only under the common terms of transi- 

 tion rocks, and grauwacke. Commencing at the base of the old red 

 sandstone, which formation he had described in the previous part of 

 the memoir, he proceeds to give an account of the underlying de- 

 posits as they succeed to each other in descending order in Shrop- 

 shire and Herefordshire. 



I. Upper Ludlow Rock. — Equivalent, Grauwacke Sandstone of 

 Tortworth, &c. 



This group, so named because the Castle of Ludlow stands upon 

 it, is as eminently characterized by the presence of organic remains 

 as the old red sandstone is by their deficiency. Amid a profusion 

 of fossils, the upper beds are characterized throughout the whole 

 range of the formation by two species of Strophomena or Leptaena, 

 an Orbicula, a plicated Terebratula, &c. all of undescribed species. 

 The middle beds contain many species of Orthocerata; Serpulae? of 

 great size, &c. ; and the lower strata are charged with a profusion of 

 small Terebratulae having a gryphoid form. Trilobites of the genera 

 Homonolotus andCalymene occur. The group has a maximum thick- 

 ness of about 1000 feet, is for the most part a thin-bedded sandstone, 

 often highly calcareous, and at other times argillaceous, and in Shrop- 

 shire frequently occupies distinct lofty ridges interposed conforma- 

 bly between the old red sandstone and the inferior limestone. 



II. Wenlock Limestone. — Equivalents, Dudley Limestone, Transi- 

 tion Limestone, &c. 



On the banks of the Severn near Wenlock, and in the Wenlock 

 Edge, this group is particularly abundant in corals and Encrinites, 

 nearly all the species of which, as well as of certain Mollusca, are 

 found in the well-known limestone of Dudley. The exact position, 

 therefore, which the latter occupies in the geological series of En- 

 gland, is thus for the first time determined. 



The upper beds rising from beneath the Ludlow rock are thin- 

 bedded and lenticular, and the lower beds in Wenlock Edge con- 

 tain many concretions of very great size and of highly crystalline 

 structure. Throughout its course in the district included between 

 the rivers Oney and Lug, this limestone is chiefly characterized by 

 the abundance of one species of Pentamerus, and at Aymestrey it 

 is rich in that and other fossils*. 100 feet are considered to be about 

 the thickness of this calcareous zone. 



• The author cannot allude to the fossils of this district without ex- 

 pressing his deep obligation to the Rev. T. Lewis of Aymestrey, whose un- 

 ceasing researches have contributed very essentially towards the zoological 

 illustration of this memoir. 



