24 EXTINCT MONSTERS 



movement becomes an upward one, then the bed of the sea is 

 converted into dry land, and the geological record is broken ; for 

 aqueous strata do not form on dry land. Blown sands and 

 terrestrial lava-flows are exceptions ; but such accumulations are 

 very small and insignificant, and may therefore be neglected, 

 especially as they contain no fossils. 



In this way, as well as by the process of " denudation " already 

 alluded to, breaks occur; and these breaks often represent long 

 intervals of time. There are several such gaps in the British 

 series of stratified rocks ; and it is partly by means of these 

 breaks, during which important geographical and other changes 

 took place, that sedimentary rocks have been classified and 

 arranged in groups representing geological periods. Thus, the 

 Cainozoic, or Tertiary, rocks of the Thames basin are separated 

 by a long " break " from those of the preceding Cretaceous period. 

 During that interval great changes in animal life took place, 

 whereby, in the course of evolution, new types appeared on the 

 scene. (See Table of Strata, Appendix I.) 



Another cause interfering with the record is to be found in 

 those important internal changes that have taken place in 

 stratified rocks often over large areas which may be ascribed 

 to the influence of heat and pressure combined. This process of 

 change, whereby soft deposits have been altered or "metamor- 

 phosed " into hard crystalline rocks, is known as " metamorphism." 

 Metamorphic rocks have lost not only their original structure 

 and appearance, but also their included organic remains, or fossils. 

 Thus, when a soft limestone has been converted by these means 

 into crystalline statuary marble, any fossils it may once have 

 contained have been destroyed. It is true that this applies more 

 to older and lower deposits for the lowest are the oldest but 

 there can be no doubt that valuable records of the forms of life 



