4 OUR COMMON BRITISH FOSSILS. 



The rocks forming the dry land are for the most part 

 of marine origin were formed along the floors of 

 ancient seas, when dry land doubtless occupied some 

 parts at least of the areas of existing oceans ; although 

 it should be remembered that modern science is 

 actively developing the theory of the relative per- 

 manency of ocean-beds and continental areas. No 

 fact is more readily or surely known than that sea 

 and land have frequently changed places. Upheavals 

 and depressions of the earth's crust, producing mar- 

 vellous physical results, and affecting the distribution 

 of life-forms in every period of our planet's history, 

 are of insignificant importance when we regard the 

 bulk of our earth as a planet. It is the sum total 

 of these depressions and upheavals, as well as of the 

 atmospherical and marine wear-and-tear of the solid 

 rocks, which has eventually given the surface of the 

 globe its present physical geography. 



Of all these things the geological student has to 

 take heed. He discovers that geology, after all, is 

 but the complete record of the physical geography of 

 the past ; and that, as Lyell and others have demon- 

 strated, the physical changes everywhere going on at 

 the present day do not differ in their nature, and 

 probably not very greatly in their intensity, from 

 those which took place in former geological periods. 

 But, undoubtedly, the nature of geological study 

 obliges the young beginner first to pick up an elemen- 

 tary knowledge of natural history. How can he 



