22 EXTINCT MONSTERS. 
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 which peopled the world in former periods 
have been lost by this means. 
And lastly, it must ever be borne in mind that, as yet, our know- 
ledge of the stratified rocks of the earth’s crust is very limited. In 
course of time, no doubt, this deficiency will be to a great 
extent made good ; but it will take a long time. Already, within 
the last thirty years, the labours of zealous geologists in the 
colonies and in various countries have added largely to our know- 
ledge of the geological record. Still, only a small portion of the 
earth’s surface has at present been explored; and doubtless one 
may look forward to future discoveries of extinct forms of animal 
and plant life as wonderful and strange as those that have been 
of late years unearthed in the “ far West,” in Africa, and India. 
The Siwalik Hills of Northern India offer a rich harvest of fossils 
