24 EXTINC!E 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 haye 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 
