6 INTRODUCTION. 



We may see, on another sea-shore, extensive beds of limestone forming from shells 

 and corals, having as firm a texture as any marble; we may watch the process of ac- 

 cumulation from the growth of corals and the wear of the waves, and find the remains 

 of corals and shells in the compact bed. If we then meet with a limestone over the con- 

 tinent containing remains of corals, or shells, no firmer, not different in composition, 

 but every way like the coral reef-rock, or the shell-rock of other regions, the mind, if 

 allowed to act at all, will infer that the ancient limestone was as much a slowly-formed 

 rock, made of corals, or shells, as the limestone of coral seas. 



In a volcanic district, we witness the melted rock poured out in wide-spread layers 

 and cooling into compact rock, and iearn, after a little observation, that just such layers 

 piled upon one another make the great volcanic mountain, although it may be ten 

 thousand feet in height. We remark, further, that the fractured crust in those regions 

 has often let out the lava to spread the surface with rock, even to great distances from 

 the opening. 



Should we, after this, discover essentially the same kind of rock in widespread beds, 

 and trace out the fractures filled with it, leading downward through the subjacent 

 strata, as if to some seat of fires, and discover marks of fire in the baking of the under 

 lying beds, wc use our reason in the only legitimate way, when we conclude that these 

 beds were thrown out melted, even though they may be far from any volcanic centre. 



If we see skeletons buried in sand and clay that we do not doubt are real skeletons 

 of familiar animals, and then in a bed of rock discover other skeletons, but of unfa- 

 miliar animals, yet with every bone a true bone in form, texture, and composition, and 

 every joint and limb modelled according to the plan in known species, we pass, by an 

 unavoidable step, to the belief that the last is a relic of an animal as well as the 

 former, and that it lies in its burial-place, although that burial-place be now the solid 

 rock. 



These few examples elucidate the mode of reasoning upon which geological deduc- 

 tions are based. 



In using the present in order to reveal the past, we assume that the 

 forces in the world are essentially the same through all time ; for these 

 forces are based on the very nature of matter, and could not have 

 changed. The ocean has always had its waves, and those waves have 

 ever acted in the same manner. Running water on the land has ever 

 had the same power of wear and transportation and mathematical 

 value to its force. The laws of .chemistry, heat, electricity, and me- 

 chanics have been the same through time. The plan of living struc- 

 tures has been fundamentally one, for the whole series belongs to one 

 system, as much almost as the parts of an animal to the one body ; 

 and the relations of life to light and heat, and to the atmosphere, have 

 ever been the same as now. 



The laws of the existing world, if perfectly known, are consequently a key to the 

 past history. But this perfect knowledge implies a complete comprehension of nature 

 in all her departments, — the departments of chemistry, physics, mechanics, physical 

 geography, and each of the natural sciences. Thus furnished, we may scan the rocks 

 with reference to the past ages, and feel confident that the truth will declare itself to 

 the truth-loving mind. 



As this extensive range of learning is not within the grasp of a single person, special 

 departments have been carried forward by different individuals, each in his own line of 

 research; for Geology as it stands is the combined result of the labors of many workers. 

 But the system is now so far perfected that the ordinary mind may readily understand 

 the great principles of the science, and comprehend the unity of plan in the earth'a 

 genesis. 



