32 FORAMINIFERA 



he might be trying to run down the form to a satisfactory name 

 even if plenty of literature were at hand. 



Vertical Ranges. As material is studied from a section, from 

 a well core, or even from carefully collected samples from a 

 standard tool well, the vertical distribution becomes known. 

 With the quick reference to the figures as the samples are 

 studied changes are easily seen and new numbered "species" are 

 added to the growing list. Occurrences are noted with records 

 as to relative abundance. 



From this data a chart may be constructed showing the ver- 

 tical distribution of the numbered "species" in the particular 

 section or well. It will be found in any section that there are 

 species which are so rare that they are of little use in determin- 

 ing the position of samples. Even if their ranges are short and 

 accurate, the time consumed in finding them in a sample and 

 the possibility of missing them make their value very slight. 

 On the other hand, there will be found species whose ranges are 

 long and which may be present through too much of the section 

 to be of use in detailed work. In every section however there 

 are species whose ranges are relatively short and which are 

 abundant enough to be quickly found in a sample if they are 

 present at all. Such species are ideal for correlation purposes. 

 If, then, these key species are selected, a chart may be built up 

 from them that will make the placing of unknown samples in 

 the section a matter of comparison. The "tops" and "bottoms" 

 of ranges may be used. If two species of different but overlap- 

 ping ranges are both present, it is at once apparent that the 

 sample that contains them came from the zone of their over- 

 lapping. By using many species, the zone of overlap becomes 

 narrowed until with rich faunas carefully worked out, the 

 accuracy of placing samples becomes one of close discrimination 

 of specific characters. Even if the zone of overlap may be of 

 considerable amount in a case where but a few species are avail- 

 able, the relative abundance of species may serve as an addi- 

 tional check and narrow down the limits. Even in similar 

 sediments the relative abundance of species at different horizons 

 is often widely different. 



Charts may be made and duplicated to give a series of work- 

 ers the data that has been worked out by the persons specializ- 

 ing in different parts of a section. One of the best of these is 



