xlviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



and Wye. The geologist who observes these tracks, their hardened 

 state from exposure to a hot sun, and the cracks in the general sur- 

 face of mud, will not fail to see the resemblance of such surfaces to 

 those laid bare in quarries, where the tracks of animals are found. 

 He has but to suppose a gradual sinking of the mass of such land 

 beneath the sea, and take into account all the modifications which 

 would arise from alterations in the heights of tide, and spread of mud 

 flats, to see how readily the results would resemble those deposits in 

 which bird-tracks have been found by Professor Hitchcock and 

 others in America, and amid the new red sandstones in Europe. 



A communication was made to you by Professor Ramsay and 

 Mr. Aveline, giving the results of examinations by the Geological 

 Survey of parts of North and South Wales, it being thought highly 

 desirable that this branch of the public service should aid the pro- 

 gress of the Geological Society whenever opportunities might occur. 

 In this communication the authors point out that, on the south and 

 south-east of the Dolgelly and Bala district, certain bands of sand- 

 stone, though comparatively of trifling thickness, are important as 

 explaining the structure of Wales. These are local and intermittent, 

 in part skirting the base of the overlying rocks through Montgomery- 

 shire and Radnorshire far down into South AVales. Above these 

 sandstones are slaty shales, 1000 to 1500 feet thick, and, resting 

 upon the latter, sandstones, mingled with occasional shales, about 

 2000 feet thick. These the authors refer to the Caradoc sandstone 

 of Sir Roderick Murchison, They support the Wenlock shales, in 

 their turn surmounted by the Ludlow rocks of Montgomeryshire. 



It is shown that the slates and associated contemporaneous igneous 

 rocks of the country north of Bishop's Castle rise from beneath the 

 Wenlock shales of Montgomeryshire, and present the same characters 

 as the slates and igneous rocks of Merionethshire, resting on the 

 purple, green, and grey sandstones of the Longmynds, which occupy 

 the same position as the Barmouth sandstones of Merionethshire 

 and Caernarvonshire. A mere trace of Caradoc sandstones is occa- 

 sionally seen between the Wenlock shales and the older rocks of the 

 Bishop's Castle district. The Wenlock shales run across the strike 

 of these older rocks. 



On the north-east of Welshpool, black slates and associated con- 

 temporaneous igneous rocks again emerge from beneath the Wenlock 

 shales. On the north of Builth the same rocks are seen lapped 

 nearly round by Wenlock shale, without the intervention of Caradoc 

 sandstone. At Llanwrtyd and Baxter's Bank, Radnorshire, similar 

 rocks rise up amid black slates, and at St. David's, Pembrokeshire, 

 contemporaneous igneous rocks with associated black slates come to 

 the surface, and repose on rocks similar to the Barmouth sandstones 

 and the old deposits of the Longmynds. 



It is incidentally noticed that *' the igneous rocks which occasion- 

 ally appear in the line of the great Shropshire and Radnorshire fault 

 are of difl'erent date and structure from those heretofore alluded to. 

 They are always massive (greenstones, syenites, &c.) ; they inva- 

 riably appear in the line of great dislocation, and alter by baking, or 



