CJiemical and Geological Esmys of T. Sterry Hani. 157 



six forinations, founded upon their fossil remains and order of super- 

 position, as follows: First — Upper Ludloiv rock (so named because the 

 Castle of Ludlow stands upon it), having a thickness of about 1,000 

 feet, very fossiliferous, and characterized by Stvophomena, Oiihocerat'des, 

 trilobites of the genera Homahnoins, Cahjmene, etc. Second — Wenloeh 

 Ihnednne, about 100 feet in thickness, and characterized by the 

 abundance of its corals, encrinites and mollusca. Third — Lotver Ludloiv 

 rock, about 2,000 feet iu thickness, and characterized by its Ortho- 

 ceratites, Litulfes, Asaphus caudatus, Penfame.rus hcvis, etc. Foui'th — 

 Shell// sandstones, from 1,500 to 1,800 feet in thickness, and charac- 

 terized by its Leptence, Spiriferi, crinoidal remains, etc. Fifth — Black 

 trilohlte flagstone, exceeding 2,000 feet in thickness, and characterized 

 by the Asaphas i^Ogygia) Bachil and other trilobites. And sixtli — 

 Red conglomnrite sandstone and slaty schist, several thousand feet in 

 thickness, in which no organic remains had then been observed. 



He regarded the deposits four, five and six, as three separate 

 formations, entirely differing from each other and from the Ludlow 

 formation, in their characters mineral and fossil, and in the distinctness 

 of their physical demarkations. 



On the 22d day of January, 1834, Mr. Murchisou read another 

 memoir upon the same subject. After having devoted another summer 

 to the study of the organic remains and order of supei-position of the 

 rocks, he divided them into five formations as follows : First — Ludlow 

 rocks, 2,000 feet in thickness; second — Wenlock and Dudley rocks, 1,800 

 feet iu thickness; third — Horderly and May Hill rocks, 2,500 feet in 

 thickness (this was made the equivalent of subdivision four in his 

 memoir of Mai-ch 27, 1833); fourth — Builth and Llandeilo flags, 1,200 

 feet in thickness ; and fifth — Longmynd and Gwadaden rocks, many 

 thousand feet in thickness. This year he accompanied his memoir 

 with a map, showing the order of superposition and the fossils so far as 

 they had been ascertained belonging to each of his subdivisions, and it 

 may be added that those most characteristic were then known and 

 mentioned by him. 



Subsequent study of the fossil contents of these several subdivisions 

 led Mr. Murchison to the conclusion that they were parts of one 

 great formation, and in honor of the ancient tribe. of Silures he pro- 

 posed for the formation the name Silurian, which he published in July, 

 1835. He divided the formation into Upper Silurian and Lower 

 Silurian rocks. The first three groups, as named in his memoir of 

 March 27, 1833, he placed in the Upner Silurian, and the fourth, fifth, 

 and sixth, in the Lower Silurian. 



The President of the Geological Society of London,, Mr, George R 



