BRITISH SILURIAN BRACHIOPODA. 67 



amounting together to 10,000 specimens at least ; but this does not nearly represent the 

 full wealth of life of this rich horizon, as many of the larger species, and others not 

 completely calcified, would get broken up in the washing process, and we have had to 

 supplement the species obtained by washing with a series of hand-picked specimens. 



" The whole of the debris has been preserved after picking out the Brachiopods, as 

 it abounds in minute Corals and other fossils, which will, we hope, be investigated by other 

 observers. 



" The cost of the process, with the aid of a potter's clay-blunging machine, amounts 

 to about 18s. per ton of materials. This includes the cartage of the shale two or three 

 miles, the whole process of washing, and the hand-picking of the fossils by paid 

 workers. 



" The following estimate of the thicknesses of the several subdivisions of the Upper 

 Silurian rocks of Shropshire is based on the average of three sections from south-south- 

 east to north-north-west, across the north-eastern end of the great Shropshire escarpment. 

 One of these passes through the town of Much Wenlock, and the others at distances of 

 about two miles to the east and west. 



" The horizontal distances of the lines of contour from the base of the Upper 

 Llandovery to the base of the Devonian average from 3| to 4 miles or about 20,000 

 feet ; and taking the general dip at 12°, the total thickness of the Upper-Silurian series 

 can scarcely be less than 4500 feet. 



" An estimate of the actual thickness of each of its subdivisions is difficult to arrive 

 at accurately, mainly from the fact that most of the zones, both in mineral character and 

 in the range of species, insensibly graduate one into the other ; and it is probable that no 

 two observers would fix exactly on the same lines of demarcation. 



" There are perhaps few parts of the country in which the surface features of 

 contours are ruled so closely by their geological structure, enabling the eye at a glance 

 to follow all the main subdivisions. Standing on Benthall Edge or Wenlock Edge, the 

 most prominent points in the escarpment, three parallel valleys and two well-marked 

 intermediate ridges can be made out at almost every part of the long line of exposure 

 extending from Ironbridge on the north-east to Ludlow on the south-west, the three 

 valleys corresponding with the soft shales and the two ridges with the limestones. 



" The broad sweeping valley of Ape Dale below the observer to the north-west 

 represents the Wenlock Shale, backed up on its north-western side by the harder beds 

 of Llandovery Limestone and Conglomerate forming the base of the Upper-Silurian 

 series. 



" The Llandovery beds on the lines of section may be roughly estimated at a 

 thickness of 160 to 170 feet, of which the conglomerate, closely resembling the 

 Milstone Grit, forms the greater bulk. The overlying Wenlock series attain a thickness 

 of from 2500 to 2800 feet; their principal mass consists of soft shales capped by the 

 Wenlock Limestone, which has determined the beautiful escarpment of Wenlock Edge, 



