Section of Geology and Geography. 483 



On Eleven Trap Dykes in the Counties of Mayo and Sligo, run- 

 ning East and West for great distances. By Archdeacon Ver- 

 schoyle. (Printed in Proceedings of Geological Society.) 



On certain Fossil Polyparia found in Alluvial Deposits in tlie vi- 

 cinity of Limestone Hills. By Dr. Jacob. 



The specimens were Lit/iodendra, of the species usual in the car- 

 boniferous limestone of England, the coralline lamellae being replaced 

 by silica, and the limestone partially removed by water containing 

 carbonic acid. Similar cases are frequent in the North of England : 

 the circumstances under which they occur appear to Dr. Jacob to 

 deserve special inquiry. 



On the Silurian and Cambrian Systems, exhibiting the order in 

 which the older Sedimentary Strata succeed each other in England 

 and Wales. By Professor Sedgwick and R. I. Murchison, 

 V.P.G.S. 



Mr. Murchison described a great group of fossiliferous deposits 

 which rises out from beneath the old red sandstone. To these rocks, 

 which he has termed in descending order the Ludlow, Wenlock, Ca- 

 radoc, and Llandeilo formations, (each distinguished by peculiar or- 

 ganic remains, and frequently by subordinate limestones,) it was 

 found essential to assign a comprehensive term, since they consti- 

 tute one natural system interpolated between the old red sandstone 

 and the slaty rocks of Wales. He observed that it was well known 

 to all practical geologists, that in consequence of the recent ad- 

 vances of the science, it was absolutely imperative that the term 

 " transition", under which such rocks would formerly have been de- 

 scribed, should now be abandoned, since it had been so used, both 

 by Continental and English writers, as to embrace the whole car- 

 boniferous series, from which the system under review was not only 

 separated by the vast formation of the old red sandstone, but was 

 specially to be distinguished by its fossil contents. Urged, therefore, 

 by many geologists to propound an entirely new name for the class 

 of rocks which had engaged his attention during the last five years, 

 Mr. Murchison recently suggested (SeeLond. and Edinb. Phil. Mag., 

 July 1835, pres. vol. p. 4-8.) that the group should be termed the 

 " Silurian System" the name being derived from the ancient British 

 people, the Silures, who under Caractacus made so noble a stand 

 against the Romans, and within whose territory the rocks under con- 

 sideration are fully displayed. Mr. Murchison then pointed out, that 

 wherever the limestones and typical characters of particular forma- 

 tions were absent or obscure, it was always practicable, over a re- 

 gion of 120 miles in length, extending from the neighbourhood of 

 the Wrekin and Caradoc hills, in Shropshire, to the west coast of 

 Pembrokeshire, to separate the groups into two parts, the " Lud- 

 low" and " Wenlock" formations, forming the "Upper Silurian" 

 the " Caradoc" and " Llandeilo" the " Lower Silurian rocks". He 

 further remarked, that in South Wales he had traced many distinct 



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