MATERIALS 27 



general rule, metamorphic rocks are unfossiliferous. 

 This is doubtless one of the chief reasons for our sparse 

 knowledge of ancient faunas, since the older rocks have 

 been involved most frequently in metamorphic stress. 



There remain the sedimentary rocks, in which alone 

 organic remains can be reasonably expected to occur. 

 As has been shown in a previous section, the three types> 

 arenaceous, argillaceous and organic, vary in quality 

 as matrices for preservation of fossils. Sands may be 

 terrestrial in origin (like some of the Old Red and many 

 Triassic sandstones), and will then prove scarcely fossili- 

 ferous owing partly to the paucity of life in deserts, and 

 partly to the imperfect burial afforded. Coarse breccias 

 or conglomerates are rarely fossiliferous ; such organic 

 remains as they enclose are usually in poor repair. 

 Fine-grained sandstones are often replete with fossils 

 (e.g. the Caradocian of Shropshire or the Thanet Sands), 

 but very little exposure to weathering reduces the remains 

 to the state of casts. 



Clays and shales, which afford the best matrices for 

 preservation, are by no means always richly fossiliferous. 

 The Wenlock Shales, Liassic clays, Gault and Barton 

 Beds may be cited as argillaceous deposits in which, 

 locally at least, rich " hauls " may be expected. But 

 against these must be placed the Old Red Marls of 

 Herefordshire, the Keuper Marls, much of the Weald 

 Clay and the Reading Beds. The scarcity of inverte- 

 brate fossils in these rock-systems is probably due in 

 large measure to the terrestrial, lacustrine or deltaic 

 conditions of their formation. The moral to be drawn 

 from the comparison is that mottled clays are rarely rich 

 in fossils. This is well illustrated in the Wealden of the 

 Isle of Wight, where grey-laminated clays are crowded 

 with Cypridea and Cyrena, while associated mottled beds 

 are practically barren. Argillaceous rocks are specially 



