230 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 21, 



Old Red, that, although this new exposure at Linley adds to our 

 knowledge of the series, it does not, in our opinion, enable us to 

 draw a more definite boundary-line between the Silurian deposits 

 and the Old Red Sandstone. 



In commenting generally upon these passage-beds, we cannot 

 choose a more fitting preface than the following passage from 

 ' Siluria ' : — 



" During the accumulation of nearly all the Silurian deposits of 

 Britain, which were characterized by a certain fauna, the bottom of 

 the sea was, to a wide extent, occupied by dark- and grey- colon red 

 sediments. At the close of that period a great change occurred, over 

 large areas, in the nature and colour of the submarine detritus. In 

 and around the Silurian region, for example, the dark-grey mud was 

 succeeded by red silt and sand, the colour being chiefly caused by the 

 diffusion of iron-oxides in the waters " (2nd edit. p. 270). 



The characters of this more or less iron- tinged zone have had the 

 careful attention of many geologists, more especially as regards the 

 fossil contents of the lowest and, hitherto, best-known layers, in 

 which the bone-bed of the Upper Ludlow shales is so noticeable a 

 feature. 



The important exposures of the uppermost division, in which 

 the rocks described in this paper occur, at and near Ludlow, and 

 at Ledbury, have revealed enough to show it to be by far the more 

 important series of the two, whether we consider it with regard to 

 its greater thickness, variety of mineral composition — indicating an 

 era of more frequent physical changes, — or for containing the remains 

 of more highly organized animals. At Ledbury, Cephalaspid Fishes 

 seem to have been the chief inhabitants of the water; while at 

 Ludlow, remains of Pterygotus and other Crustaceans occur plenti- 

 fully in and near this " upper bone-bed." At Linley, remains of 

 those Fishes known as Plectrodus, Onchus, Otenacanthus, &c, charac- 

 terize the zone, and no Crustacean remains have yet occurred. 



The position of the beds at Linley is shown by sections along 

 the course of Linley Brook, a stream which falls into the Severn at 

 Apley, four miles north of Bridgnorth. Opposite Linley Hall the 

 Upper Ludlow rock is exposed in the sides of a little gorge formed 

 by the stream, the dip being S.S.W. Here it is traversed by two 

 sets of joints, one N.E. to S.W., the other W.S.W. to E.N.E. Lower 

 down the brook, near the " Holly Bush," a more extended section 

 may be obtained. The dip is here 15° to the S.E. 



The series exposed in this sinuous gorge ranges from the true 

 Aymestry rock, which forms, in places, the bed of the brook, to the 

 Upper Coal-measures, capped with northern drift and scratched 

 boulders. The coal-seams are but two in number, and of poor 

 quality. . One or more beds of jointed freshwater limestone lie 

 among the clays. The exact junction of these coal-measures with 

 the Old Red we failed to discover, though at a point high up the 

 north bank, near the " Holly Bush," the line of fault marked down 

 by the Geological Survey, as dividing the two systems, is traceable 

 in the steeper inclination of the Coal-measure clays. 



