Mr. Murchison on the Silurian System of Rocks. 49 



bracing the deposits of " Ludlow" and " Wenlock," the latter 

 those of " Caradoc" and " Llandeilo." By this arrangement 

 the observer will not be embarrassed when he finds that 

 certain typical strata have disappeared. Thus, for instance, 

 when the zones of limestone thin out, by which the forma- 

 tions of Ludlow and Wenlock are subdivided, it is no longer 

 practicable to mark lines of separation between them. Under 

 such circumstances the united mass will simply be described 

 as " Upper Silurian rocks" whilst, wherever similar causes 

 prevent the separation of the M Caradoc sandstones" from 

 the " Llandeilo flags," the two will be termed " Lower Silu- 

 rian rocks" 



A wood-cut is annexed, explanatory of the manner in which 

 these deposits are connected with the old red sandstone above 

 them, with each other, and with the slaty grauwacke rocks 

 beneath them. Although the lower Silurian rocks are fre- 

 quently unconformable to the schistose grauwacke, as repre- 

 sented in the wood-cut, there are situations in which a passage 

 from the one to the other may be detected. 



Having alluded to a work which is in preparation upon 

 the geology of the country of the Silures, and of several 

 of the surrounding counties, I may take this opportunity of 

 acquainting those friends who have fostered the undertak- 

 ing, that the author is unceasingly occupied in promoting 

 its completion. Geologists are not, I am sure, among those 

 who feel surprise at the delay ; for they well know that so 

 large an area of country, and one so little previously ex- 

 amined, could not have been adequately described without se- 

 veral years of hard labour. In the mean time 1 may state 

 that a map is now constructing, being a precise reduction of 

 the Ordnance Maps, extending over the greater part of ten 

 counties, all of which have been geologically coloured in the 

 field ; together with about twenty plates of engravings of fossils, 

 many of which are of new species, and a vast number of co- 

 loured sections, vignettes, &c, all of which will, I trust, be 

 completed so as to enable me to lay the work before the public 

 in a few months. 



P.S. Authorities differ in opinion concerning the exact 

 geographical boundaries of the kingdom of the Silures. 

 Cluverius and Camden state that it embraced the greater 

 part of South Wales, including also Monmouthshire and 

 Herefordshire. Hume speaks of the Silures as a nation in- 

 habiting the banks of the Severn, whilst others think that 

 their chief river was the Wye, The Roman historians afford no 

 correct account of the geography of this region, but they as- 

 sure us that the Silures were, of all the nations of South Britain, 



Third Series. Vol. 7. No. 37. July 1835. H 



