part 3] silubiax kocks of the clux-fokest district. 229 



In this connexion Murchison 1 notes that 



' Like other Silurian sediments of higher antiquity, this mudstone has a 

 tendency to run into large spheroids, and occasionally contains small concre- 

 tions of sandy clay, which, being more destructible than the pure argillaceous 

 matrix, weather out. . . .' 



It is noticeable that the weathering varies according to the 

 location of the rock. Where exposed on hillsides, it remains blue- 

 grey on freshly-broken surfaces ; but, when protected by a layer 

 of soil or subsoil, it weathers to that pale yellow-brown which is 

 so well seen in the sections at Norton Camp near Craven Arms. 2 

 In the latter case the fossils are preserved as iron-stained casts. 

 Similar differences in weathering may be noted in the ffliynchonella 

 Flags, but they are not quite so marked. 



The brownish weathered rock somewhat resembles in appearance 

 a soft sandstone, and this resemblance seems to account for the 

 fact that several writers have referred to ' brownish sandy shale ' and 

 ' brown flagstones and sandstones ' succeeding the Lower Ludlow 

 Shales in Clun Forest. It is difficult to find a suitable name for 

 the beds which have here been termed ' calcareous flags ' and ' shales.' 

 Murchison 3 refers to 'oTevcalcareo-arsnllaceous masses .... which, 

 from their incoherent nature, easily decompose into mud ' ; he 

 notes that ' the chief .... portion of the Upper Ludlow contains 

 more calcareous matter and sand than the beds immediately beneath,' 

 and compares the rock with the Tertiary ' macigno ' of Italy. A 

 microscopical investigation of specimens from the Dayia Shales, 

 the ffliynchonella Beds, and the Chonetes Beds has shown that the 

 rocks consist largely of fine quartzose silt ; sand-grains are few or 

 absent, flakes of mica fairly abundant, and the matrix consists of 

 calcareous and argillaceous matter. 



Throughout the Chonetes Beds occur bands of soft o-rev shale 

 from 3 to 12 inches thick. Two such bands are exposed in the 

 gorge of the Redlake above Llynaven Farm. 



Fossils are far more numerous in the Lower Chonetes Beds 

 than in the ffliynchonella Beds, and these increase in numbers 

 towards the top of the series. The most characteristic forms are 

 Chonetes striatella. Discina rayata, ffliynchonella nucula, and 

 Orthis lunata. The chamber-casts of Orthoceras mentioned in 

 the account of the ffliynchonella Beds also occur, but are less 

 common. Numerous species of lamellibranchs occur, but Orthonota 

 amygdalina of lower beds is less conspicuous. Serpulites Jonyis- 

 simas, too, is less common in the Chonetes Beds than in the 

 ffliynchonella Beds. 



In the upper part of the Lower Chonetes Beds crinoid-stems and 

 impressions of ossicles are frequently abundant, and often associated 

 with casts of bryozoa. The zone-fossil, Chonetes striatella. 

 varies considerably in its specific characters. In the fossil lists 



1 ' Siluria : 4th ed. (1867) p. 131. 



2 G. L. Elles & I. L. Slater, Q. J. G. S. vol. lxii (1906) pp. 214- 18. 



3 Op. supra cit. p. 131. 



