TEANSACTIONS OF SECTION C. 425 



Series, and it is noteworthy that the granophyi'es appear to be closely related to the 

 extrusive rhyolites among which they are intruded. This fixes an upper limit to 

 the age of the diabases in this district. 



The various beds strike more or less east to west, and dip steadily southwards 

 at about 40° until the Talyllyn Mudstones are reached, when folding and rolling of 

 the beds immediately begin. Two N.E.— S.W. shatter faults— the Dolgelley and 

 Talyllvn faults — cause a certain amount of repetition, and give rise to the Dolgelley- 

 Llyn Gwernon and to the Talyllyn Valleys, the former to the north and the latter 

 to the south of the escarpment. A strike-fault between Mynydd Gader and Cader 

 Idris cuts out the whole of the Bifidus Beds, bringing the Lower Acid and the Lower 

 Basic Volcanic Series against one another. The intrusive rocks frequently cause 

 local variations in the dip and strike. 



All the softer strata are strongly cleaved, so that fossils are difficult to obtain. 

 The slates within the Lower Acid Series have yielded a few extensiform graptolites, 

 while from the Bifidus Beds the characteristic fossils Were obtained at numerous 

 localities. The D. murcMsoni zone has not been recognised by the authors, its place 

 presumably being occupied by a part of the Lower Basic Series. The dark mud- 

 stones among which the pisolitic iron-ore is developed have yielded rather obscure 

 graptolites, which, however, indicate a fairly high horizon in the Llandeilo Series. 

 The presence of Amplexograptus arctus and Olijplogrci'ptus teretivscvhis var. euglypJnis 

 in the lowest beds of the Talyllyn Mudstones indicates a high horizon in the Glenkiln, 

 or, in other words, a low horizon in the Caradocian, and suggests that the immediately 

 underlying Upper Acid Series is at approximately the same horizon as the Snowdonian 

 volcanic rocks of Conway. This youngest of the four volcanic series on Cader Idris 

 is therefore considerably higher in the Ordovician than has been previously supposed. 

 The position of the boundary between Caradocian and Llandeilian has not yet been 

 established, owing to the unfossiliferous character of the blue-grey mudstones of 

 Llyn-y-Gader and Llyn Cau. 



One of the authors fA. H. Cox) is indebted to the Government Grant Com- 

 mittee of the Royal Society for a grant which has partially defrayed the expenses 

 involved in the investigation. The area is being mapped on the 6-inch scale. 



4. On the Underground Contours of the Barndey Seam of Coal in the 

 Yorkshire Coalfield. By Professor W. G. Fearnsides, M.A. 



In this paper the author presents a preliminary account of the results of his 

 statistical analysis of about a hundred records of borings and sinkings which 

 have proved the depth of the Barnsley Bed or its equivalents (the Gawthorpe, 

 the Warren House Coals of Yorkshire, and the Top Hard Coal of Derbyshire) 

 in Yorkshire. The majority of the records of borings and sinkings discussed 

 have been collected by a committee of the ' Midland Institute of Mining, Civil 

 and Mechanical Engineers,' published by that Institution in volume form in 

 1914, the sites at which the information was obtained being plotted on a half- 

 inch map. The depths to the coal have been corrected for the height of the 

 surface location above sea-level, and, after the manner of Dr. Gibson's map 

 (Plate 1) in the ' Geological Survey Memoir on the Concealed Coalfield,' con- 

 tour lines have been drawn among the spot-levels so obtained. Other contour 

 lines similarly obtained from the records of borings which have passed through 

 Permian strata show the character of the surface of the Coal Measures where 

 they underlie the Permian strata. 



In drawing the contour lines no attempt has been made to distinguish 

 between those changes of level in the seam between neighbouring pits which 

 are due to faulting and those due to the folding of the strata. Since, however, 

 over most of the coalfield the faults tend to nullify the change of level which 

 the dip has accomplished, it is maintained by the author that to plot contours 

 which show the average rate of change of level is a statistical process which can 

 be demonstrated as approximating to truth. 



(1) From an analysis of the results as plotted it appears that the under- 

 ground contours in the Barnsley Bed (strike lines) in Yorkshire in detail 

 generally range either N.E.-S.W. or N.W.-S.E., and that within the area under 

 which the Barnsley Bed has actually been proved by working it is difficult 

 to find either a N.-S. or an E.-W. strike constant over more than a few miles 



