22 AN INTRODUCTION TO THE STUDY OF FOSSILS 



parallel with the bedding of the strata, whereas the slickenside 

 will be seen usually to cross successive laminae or even beds. 

 Sometimes fractures, especially in dense, fine-grained rocks, re- 

 semble the external molds of pelecypods or other bivalve shells. 

 An inorganic form very often mistaken for true organic remains 

 is the dendrite (Fig. 6) ; this branching incrustation, formed 

 usually of manganese oxid, is common, either inclosed, as in the 

 moss agate, or merely as surface markings. It looks much like 

 a piece of moss or fern, but there is no regularity in its method 

 of branching, whereas an organism always exhibits a regularity, 

 a symmetry. 



Collecting fossils. — In compact, semicrystalline rocks the 

 fossils upon an exposed surface, especially under a residual clay, 

 are apt to become silicified, and hence when entirely weathered 

 out of the rock they form ideal specimens. This weathering 

 out of the entire compact fossil is especially characteristic of 

 shales, shaly limestones, and of marls, consolidated or uncon- 

 solidated. Hence excellent places for collecting fossils are 

 along weathered rock surfaces, water courses leading from 

 such rocks, or even in plowed fields, especially immediately after 

 a rain. 



Index fossils. — Every fossil is more or less an index to the 

 age of the rock in which it occurs, for it is a relic of the life which 

 inhabited the earth when that sediment which now forms the 

 inclosing rock was being deposited. It was early observed that 

 succeeding rocks contain different fossils, that as they were 

 followed from the lower to the higher beds, the inclosed fossils 

 changed. At present the succession of life in general upon the 

 earth is known, though more and more of its details are being 

 discovered each year. It is known that these successive faunas 

 and floras follow each other in the rocks the world over in 

 approximately the same order. 



It has been observed that each fossil is not of equal impor- 

 tance as an index to the age of a rock ; some species, such as 

 the brachiopod, A try pa reticularis, occur in the strata of two 



