PALEONTOLOGY 857 



relative length of the life of the race in question is shown by its 

 vertical extent in the sediments. A short-lived species left its fos- 

 sils in a very few feet of sediments ; others are recorded in hundreds 

 of vertical feet ; others in literally thousands ; and a few species 

 (of Globigerina) have lived uninterruptedly from the Cretaceous 

 period to the present, representing many miles of vertical extent in 

 the sediments. 



"Whether dealing with larger fossils or with microscopic ones, 

 certain features are desirable when doing geologic work. An ideal 

 fossil species: (1) had a short species life; (2) should have left 

 abundant fossils, since a species which sharply marks off a narrow 

 part of the column is valueless if it is very difficult to find; (3) 

 should have had wide distribution, leaving its record over a large 

 area ; (4) should be readily recognizable, requiring a minimum of 

 time-consuming measures, such as the counting of plates and spines 

 or measurements of different features. 



The only fossils which completely satisfy the above-mentioned 

 requirements are the ammonites (Fig. 148). The various species 

 were short-lived; in some cases the fossil specimens are actually 

 thicker than the sediments which were laid down during the life 

 span of the species. Most ammonite species occurred in large num- 

 bers, literally dominating the seas. Most ammonites had a wide 

 distribution, sometimes world-wide. Often the chambered shells, 

 comparable to those of nautili, acted as airtight bulkheads after the 

 death of the animals, and the shells floated to great distances. Thus 

 they dated sediments of the same time, whether the animals lived 

 in the waters above any particular locality or not. Finally within 

 a region, most of the ammonite species are characteristic and easily 

 identified. 



But ammonites are valuable only when working with rocks ex- 

 posed at the surface and only in a limited part of the geologic 

 column. 



Efforts have been made, naturally, to use less perfect fossils, and 

 methods have been developed to overcome difficulties when the 

 species concerned lack one or more aspects of the ideal set forth 

 above. 



Since too great a vertical range is the most serious difficulty en- 

 countered in establishing precise levels in the strata, a brief sum- 



