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 "metamorphosed" 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 knowledge 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 knowledge 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 to future explorers. Already, one remarkable and large horned quadruped has come from this region; and it is known that other valuable treasures are sealed up within these hills, only awaiting the "open sesame" of some enterprising explorer to bring them to light. As previously pointed out, deposits formed in lakes are the most promising field for geologists in search of the remains of old terrestrial quadrupeds and reptiles; but, unfortunately, such deposits are rare.