46 REPORT — 1844, 



however, goes on more slowly now, partly from the mucli wider extent of the falls 

 weakening the force of the water at any one point, and partly, the author imagines, 

 from the volume of water having diminished. 



In conclusion, the author thinks it possible that even in our own island we are not 

 precluded from supposing that the same causes may have excavated river channels, 

 since it may be considered that England was at one time a portion of a great con- 

 tinent. 



On the Midland Coal Formations of England. By Elias Hall. 



An Account of that Portion of the Ordnance Geological Map of England now 

 completely coloured, and Azotes concerning a Section through the Silurian 

 Rocks in the vicinity of Builth, By Sir H. T. De la Beche, F.R.S., S)-c. 



The author gave an account of the method adopted in pursuing the geological 

 survey of England, and the natui-e and degree of accuracy of the maps and accom- 

 panying sections. He then stated that the vicinity of Builth is one of much geological 

 interest, as showing the connexion between the Silurian rocks at Ludlow, Wenlock, 

 and other localities on the N.E., with the same deposits in Brecon, Carmarthen, &c., 

 and as affording considerable instruction relative to the intermixture of sedimentary 

 and igneous rock at this early period. The section described was part of one now 

 making by the Geological Survey between the old red sandstone of the Black Moun- 

 tains in Brecon and the sea north of Aberystwith. Sir H, De la Beche then com- 

 pared this development of the Silurian rocks with that in Siluria, and observed, that 

 although there is but a trace of the Wenlock and Aymestry limestones near Builth, 

 still there is a general resemblance to the sequence described by Mr. Murchison at 

 Malvern, Woolhope, &c. It is at the base of the Wenlock shales that the greatest 

 modification is found ; instead of the Caradoc limestone and sandstone are the shales 

 and slates with Asaplnts Buchii, and beneath these a mixture of conglomerates, sand- 

 stones, &c., with similar fossils ; so that either the sandstones representing the Caradoc 

 are included in the Llandeilo flags, and one appellation must be applied to both, or the 

 Caradoc sandstone must be supposed to have thinned off, so as not to occur in the 

 Builth and western sections. 



On certain Silurian Districts of Ireland. By Richahd Griffith, F.G.S. 



In this communication, Mr. Griffith first noticed the occurrence of Silurian fossils in 

 two extensive districts in Ireland, which have been examined by him during the 

 period which has intervened since the Meeting at Cork. One of those districts is 

 situated on the west, and the other on the east coast. That on the west was stated to 

 occupy a considerable portion of the counties of Mayo and Galway, to the north and 

 south of the remarkable estuary called Killery Harbour. 



This district is bounded on the north by the mountain range of Croagh Patrick in 

 Mayo (which is chiefly composed of mica slate), and on the south by the primary 

 mountain group, called the Twelve Pins of Connemara, in the county of Galway. 



Mr. Griflith exhibited a detailed section of the strata extending from south to north 

 from Galway Bay, across the western portion of the group of the Twelve Pins, and 

 thence by Killery Harbour towards Croagh Patrick. The district immediately to the 

 north of Galway Bay consists of sienitic granite, and occupies a tract of country ten 

 miles in breadth. It is succeeded on the north by a metamorphic district, (consisting 

 of imperfectly stratified rocks, presenting the characters of imperfect gneiss, horn- 

 blende slate, and semi-porphyry, having sometimes a siliceous, and sometimes a horn- 

 blendic base.) which occupies a stripe of country varying from two to six miles in 

 breadth. Beyond is the central group of the Twelve Pins, which is composed of alter- 

 nations of mica-slate, white quartzite, and primary limestone, the mica-slate predomi- 

 nating. On the summit of Benbawn, quartzite reaches an elevation of 2395 feet. 



The limestone beds which alternate with the mica-slate, frequently present a cr}-- 

 stalline structure, and pass into granular marble ; and in several localities, but parti- 

 cularly in the valley of Barnanoraun, north of Ballinahinch, there are thick beds of 

 yellowish-green steatitic marble, altei-nating irregularly with bands of limestone of 

 various shades of colour (Connemara marble). 



