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 



