MATERIALS 35 



Earthquakes and volcanic eruptions would hardly work 

 such havoc upon Invertebrates as they often produce 

 among fishes ; but if they caused upheaval or other 

 geographical changes, their sudden nature would be 

 fatal to sedentary forms. It is possible that some of the 

 fossil-bands of the uppermost Silurian, notably the Bone 

 Bed and Lingu/a-bed before mentioned, may be due to 

 seismic catastrophes. 



The clearest and most frequent types of fossil-bands 

 are found in strata of the littoral zone. In most of such 

 cases the shell-beds contain a variety of forms, although 

 one or two species usually predominate. Almost all the 

 fossils are worn or broken, and the matrix may be full 

 of shell-fragments. These fossil-bands can be ascribed 

 to tidal currents or waves, sorting out dead shells of 

 similar shape and weight, and drifting them to the 

 beach or sheltered water. " Shell-sand " is a common 

 product of this action. On sandy beaches it is quite 

 usual to find abundant shells after continued winds from 

 a particular quarter, while at the next visit nothing but 

 sand and shingle can be seen. If isostatic conditions 

 are such that permanent sedimentation is in progress, 

 such episodes will be perpetuated as alternations of fossili- 

 ferous and barren deposits. 



Lastly, reduction in the rate of accumulation of sedi- 

 ment, due to conditions that do not seriously affect the 

 local fauna, naturally increases the relative number of 

 fossils in a given mass of rock. Development of organic 

 oozes on the floor of the open sea, beyond detrital limits, 

 gives an extreme example of this type of " fossil-band." 

 Local (usually temporary) circumstances may give com- 

 parable results in coastal deposits. Illustration of such 

 conditions is found in the Inferior Oolite. In Dorset- 

 shire, where, owing to irregularity and hindrance in 

 isostatic depression, rocks of this system rarely exceed 



