26 INVERTEBRATE PALAEONTOLOGY 



(among other debris) and enclose them on crystalliza- 

 tion. Such a process usually results in utter destruction 

 of organic matter ; but sometimes the lava consolidates 

 before the whole fabric is consumed, thus perpetuating 

 an impression of its form. Even ears of corn have been 

 found "fossilized" in recent lava -streams. However, 

 occurrences of this nature are so rare that magmatic 

 rocks may be dismissed as practically unfossiliferous. 



The products of volcanic explosion fall into a different 

 category. While coarse blocks of agglomeratic material 

 are likely to crush organisms beyond recognition, the 

 finer "ash" or tuff settles so gently (especially under 

 water) that it is no more destructive than clay or ooze. 

 Since most tuffs that are geologically persistent are those 

 erupted under, or carried into, the sea, they resemble 

 sedimentary rocks in accumulation, and hence may 

 include fossils. Although the immediate vicinity of a 

 volcanic vent would soon become destitute of life, 

 regions far removed from the danger-zone receive great 

 quantities of fine ash, transported peaceably by currents. 

 Thus, while tuffs associated with agglomerates and lava- 

 flows are usually barren, those interbedded with ordinary 

 sediments are often fossiliferous. 



Metamorphic rocks result from recrystallization of 

 other types under the influence of heat or crustal 

 movement. Even those schistose varieties whose sedi- 

 mentary origin is probable have undergone much 

 molecular readjustment, so that the original character 

 of the rock and its contents is altered. But meta- 

 morphism varies in degree, and such rocks as clay- 

 slates, in spite of cleavage and incipient crystallization, 

 often contain recognizable traces of organic remains. 

 These are usually much distorted (see PI. iii. fig. 3) ; 

 indeed, the state of the fossils was one of the phenomena 

 that proved the tectonic origin of slaty cleavage. As a 



