MATERIALS 47 



during life, or cause its death, have to be added incal- 

 culable, but always serious, risks of post-mortem damage. 

 The majority of shells are broken before they reach 

 the sanctuary of a sedimentary grave. The scour of 

 waves or insidious attacks of the weather, and the 

 unsympathetic investigations of carrion-feeders, will 

 pass over few of the organic remains exposed to them. 

 Sedentary animals find congenial foundations on empty 

 shells, while those of retiring habits welcome the soluble 

 fabric for excavation of their crypts. 



Fossils embedded in sandstones or limestones are 

 liable to be crushed unless their cavities are filled with 

 sediment or concretionary matter. Brachiopods and 

 Irregular Echinoids are especially affected in this way, 

 owing to the small size of their foramina and consequent 

 imperfect infiltration of sand or ooze. But in general 

 fossils in such matrices do not suffer mechanically 

 after entombment. In argillaceous rocks, on the other 

 hand, no hollow shell is safe (however well filled with 

 clay) unless a concretion has developed within it. The 

 pressure that converts clay to shale works equally on 

 the materials inside and outside the fossils, so that the 

 latter are inevitably flattened. The more severe stresses 

 and strains that produce slaty cleavage will involve the 

 fossil-contents in shearing ; so that the already flattened 

 shells take on lateral distortion. At some horizons in 

 the ZzVzg'w/tf-Flags, Lingulella davisi may be collected 

 in great abundance, but hardly two specimens out of 

 hundreds will have the same shape. Even when 

 cleavage is incipient, fossils are distorted (see PI. iii. 

 fig. 3), while development of complete slatiness usually 

 renders organic traces practically unrecognizable. 



In slowly forming sediments, empty shells become 

 encrusted by organisms of sessile habit. Frequently 

 Hydrozoa, Polyzoa and Worms grow on the surfaces 



