DETERMINING CHRONOLOGY 347 



Several scantily soluble substances act as petrifying materials, the 

 most perfect results being given by silica. A silicified bone, or 

 tooth, or bit of wood, differs from the original only in weight, 

 colour, and hardness, and when a thin section is examined under 

 the microscope, the minutest details of structure may be made out 

 as perfectly as from the unaltered original. CaC0 3 is a very 

 common petrifying agent, but it often obliterates structure by 

 crystallizing after deposition ; less usual are iron pyrites and 

 siderite. It need scarcely be said that only hard substances — 

 wood, bones, shells, etc. — are sufficiently durable to be preserved 

 in this way, though a petrifaction of soft tissues has been reported 

 as occurring under very exceptional circumstances. 



II. What may be learned from Fossils 



The principal value which fossils possess for the geologist lies in 

 the assistance which they give him in reconstructing the history of 

 the globe. This they do in several ways. 



(1) In determining Geological Chronology. — The most obvious 

 way in which to make out the relative ages of a series of stratified 

 rocks is to determine their order of superposition, for the oldest 

 will be at the bottom and the newest at the top (see p. 221). But 

 this method is of only local application and will not carry us far in 

 an endeavour to compile a history of the whole earth. It cannot 

 enable us to compare even the rocks of different parts of the same 

 continent, for any exposed section is but a small fraction of the 

 whole series of strata. More embarrassing still, strata change their 

 character from point to point, limestone being laid down in one 

 place while sandstone is accumulating in another. Still less can 

 the order of superposition help to determine the relative ages of 

 rocks in different continents, for this order in North America can 

 be no guide to the succession in Africa or Australia. This conclu- 

 sion does not imply that order of superposition may be safely 

 neglected ; on the contrary, it is an indispensable aid, but it must 

 be studied in connection with the fossils. 



Since the first introduction of life on the globe it has gone on 



