MATERIALS 33 



In most cases restriction of fossils to thin, often imper- 

 sistent, layers is not associated with any marked changes 

 in the matrix. Even so homogeneous a rock as the 

 Upper Chalk may show discontinuity in its fossil 

 contents, but the feature is most common, and most 

 striking, in littoral or shallow-water deposits. The 

 Ordovician rocks of Shropshire illustrate the phenomena 

 in a remarkable manner (PL vi. fig. 2). The more 

 arenaceous portions, such as the Horderley Sandstone or 

 Acton Scott Beds, consist of alternations of hard, flaggy 

 sandstone almost devoid of fossils, with thin, impure lime- 

 stones (becoming " rotten " on weathering) in which fossils 

 are so crowded thatdetrital matter is subordinate. Again, 

 the Onny Shales are famed for their abundant Trinudeus ; 

 but, unless the right layer is located, search will be 

 scantily rewarded. The Upper Silurian rocks of the 

 same district show similar qualities. Matted layers of 

 Orthis and Chonetes, only one shell thick, are separated 

 by considerable masses of sparsely fossiliferous Upper 

 Ludlow flags (PL vi. fig. i), while the comparably con- 

 gested band of Lingula cornea that underlies the Old 

 Red Marls is isolated in a great bulk of barren micaceous 

 sandstones. Mention may be made of the classic 

 " Ludlow Bone Bed," where Vertebrate remains occur 

 under similar conditions. In rocks of much later 

 formation the same feature is found. Visitors to White- 

 cliff Bay, in the Isle of Wight, can hardly fail to notice 

 the thick layer of white shells (chiefly Cardita and 

 Turritella) which stands vertically in the cliffs of dark 

 Bracklesham clay ; closer inspection shows that fossils 

 of the Tertiary deposits in the island are usually dis- 

 posed in bands. 



It is difficult to give a general explanation of this 

 phenomenon ; each case will have its determining causes, 

 and these may or may not be apparent. However, it 

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