PRINCIPLES OF GEOLOGY. 11 



uniform in all parts of the world, as to allow no doubt of their cotem- 

 poraneous origin, and even in some of their subordinate beds an astonish- 

 ing affinity prevails ; but neither these beds, nor the formations them- 

 selves, are in all places, continuous in extent, nor constant in thickness, 

 nor identical in chemical composition. Hence arise differences in the 

 geological structure of different countries, and the more distant from 

 each other the points of comparison, the less perfect is the agreement 

 of the rocks. Some writers, overlooking these differences, have errone- 

 ously asserted the universality of formations ; others, not understanding 

 the simple and beautiful gradations of nature, have absurdly denied the 

 regular construction of our planet. 



THE ORGANIC FOSSILS commonly found in the earth are plants, corals, 

 crusts of radiaria, shells of mollusca, the hard coverings of Crustacea, 

 scales and bones of fishes ; reptiles, cetacea, birds, and mammiferous quad- 

 rupeds. From very early times, the wonder of mankind has been excited 

 by those fossil shells which are inclosed in rocks, and buried in mountains, 

 far removed from the sea. To find the cause of this phenomenon was 

 an object of interest, long before any settled system of geological observa- 

 tion and induction was thought of. The study of organic fossils was pro- 

 secuted with various success by different naturalists, but it was reserved 

 for our own times to demonstrate their high importance in elucidating 

 the history of the earth. Undoubtedly it is possible to acquire a com- 

 petent notion of fossil plants and animals, without particular reference 

 to geology ; but no one can be a geologist who disregards the natural 

 history of fossils. 



The slightest practical examination of rocks demonstrates that whilst 

 some strata abound with these remains, others contain very few, and 

 some are absolutely void of them. The absence of fossils was once used 

 as a character of the primary rocks, but incorrectly ; for several of the 

 secondary, and all of the independent rocks, are as destitute of fossils as 



c 2 



