50 



described a heterogeneous assemblage of plants and shells both of 

 fresh- and salt-water species. A band of ironstone, nearly in the 

 centre of this series, contains four genera of Trilobites: in the same 

 coal-field Mr. Anstice has recognised two genera of insects. On 

 the opposite side of the Severn, Mr, Murchison has found at Pontes- 

 bury, Uffington, Le Botwood and other places, a band of compact 

 limestone, between two beds of coal, resembling the lacustrine 

 limestone of central France, and containing freshwater shells. These 

 discoveries may throw light on those which have been since made at 

 Burdie-house and elsewhere in the neighbourhood of Edinburgh. 



The structure of other coal-fields has been illustrated by Mr. 

 Murchison, Mr. R. J. Wright, and Mr. England. 



After careful examination of the Old red Sandstone, Mr. Murchi- 

 son has proposed to divide it into three parts : the uppermost, cha- 

 racterized by quartzose Conglomerate ; the middle, by Cornstone; 

 the lowermost, by Flagstone. The cornstone and maristone of the 

 middle group contain undescribed genera of Crustacea ; and in the 

 tilestone beneath are found some defences of fish, together with a 

 few remains of testacea. 



Mr. Murchison has employed three summers in examining a range 

 of country situate between Shrewsbury and Caermarthen; and the 

 geological positions as well as the mineral and zoological characters 

 of the several rocks which border England and Wales are now deter- 

 mined with as much exactness as those of any portion of the secon- 

 dary system. Taking the old red sandstone as a line of departure, 

 the rocks beneath are disposed in descending order as follows: 



1. The Ludlow series, divisible into three parts, the upper, 

 middle and lower. To the middle belong the well-known limestones 

 of Amestry and Sedgley : the upper and lower consist of sand, marl, 

 or flagstone, iiaving some fossils peculiar to each, and others in 

 common. The thickness of the whole is estimated at 1000 feet. 



2. The Dudley or Wenlock series, consisting of limestone : its 

 thickness may be taken at 2000 feet. 



3. The Hordesley or May Hill series, composed of party-coloured 

 sandstone, conglomerate and impure calcareous flagstone : it is said 

 to attain a thickness of 2500 feet. 



4<. The Built or Llandilo series, a black flagstone, characterized 

 by the Asaphus Buchii. 



5. The Longmynd or Linley series, consistingof coarse roof slate, 

 sandstone and conglomerate ; no fossils have been discovered in it. 



It is well known that Professor Sedgwick has studied with equal 

 assiduity the rocks which lie beneath those I have mentioned. 

 When his observations are published, the Society will have a type 

 of the whole of the transition rocks of Wales. The rocks described 

 by Mr. Murchison are, for the most part, exceedingly well charac- 

 terized by their fossil contents. Some of the shells which he has dis- 

 covered, appear to have escaped the notice of antecedent observers; 

 but the genera, if not the species, of others, may occasionally be found 

 in the works of Hisinger and other continental writers. If, then, the 

 transition as well us the secondary and tertiary beds can be identified 



