68 SUPPLEMENT TO THE 



overhanging the gentle sweep of Ape Dale, and on the south side forms a regular dip 

 slope into the lower Ludlow valleys of Much Wenlock and Hope Dale. 



" No clear line of boundary exists between the limestone and the shale ; for the one 

 imperceptibly graduates and dies out into the other. 



" From careful measurements made on Benthall Edge I have ascertained that the 

 compact limestone is from 80 to 90 feet in thickness ; and it thickens somewhat in 

 the direction of Wenlock to the south-west. Below the compact limestone the rock 

 becomes interstratified with thin layers of shale. Still lower down it assumes a 

 concretionary structure, and gradually dies out into soft shale, through increasingly 

 distant nodular courses, at about 400 or 500 feet below the crest of the limestone 

 ridges. 



" On Benthall Edge the Wenlock Limestone dips from 15° to 20° south-south-west; 

 to the westward the dip decreases to from 10° to 15°, and at the eastern extremity of the 

 escarpment at Lincoln Hill, near Coalbrook Dale, the inclination increases to from 45° to 

 50°. The upturning may have been continuously gradual or interrupted. It commenced 

 before the Carboniferous period, for the Coal-measures rest upon it unconformably ; and it 

 continued subsequently, as indicated by the fact that the inclination of the margin of the 

 Carboniferous beds is related to the greater or less inclination of the subjacent Wenlock 

 Limestone. 



" The following proposed subdivision of the great mass of Wenlock Shale, which at the 

 north-eastern end of the Shropshire escarpment attains a thickness of from 2000 to 2200 

 feet, has been suggested by the alternation of zones of highly fossiliferous and of com- 

 paratively barren strata. 



" As stated above, there is an insensible gradation between the Wenlock Limestone 

 proper and the Wenlock Shale, the shales under the limestone containing scattered con- 

 cretionary courses of nodular limestone ; and it will be convenient to term this inter- 

 mediate zone ' The Tickwood Beds,' which may be roughly estimated to include a 

 thickness of from 300 to 500 feet of strata. 



" They are exposed in the deep road-cutting near the railway-bridge between 

 Tickwood and Parley Dingle. There is also a fine natural exposure 2^ miles to the east, 

 by the side of a small stream flowing down the east end of Benthall Edge, opposite 

 Ironbridge; and most of the adjacent cutting on the Severn-Valley Railway passes 

 through the base of these nodular limestones and shales. The Tickwood Beds are highly 

 fossihferous. They contain all the five species of Spirifera found in the Upper Silurians of 

 Shropshire, with a larger proportion of individuals than in any other zone. The 

 Tickwood Beds are also the highest horizon in which the new genus Glassia occurs ; and 

 Orthis hiloha here attains its highest limit, with the exception that a few individuals occur 

 rarely in the Wenlock Limestone and Lower Ludlow. 



"Below the fossiliferous Tickwood Beds, from 1800 to 1900 feet of soft shales occur, 

 which are comparatively barren in organic remains, and the few individuals that occur 



