34 FORAMINIFERA 



a visual chart which is really a key to the particular section. 

 See figure. For the first division two very different forms each of 

 which is common yet limited to its own part of the section should 

 be chosen. The left side of the chart may thus be divided into 

 Divisions 1 and 2, those of NodosmHa Xy-1 and Discorbis Xy-1. 

 The upper portion, Division 1, may be further divided by Nonion 

 Xy-1 and Textularia Xy-1. The zone of Nonion Xy-1 may be 

 subdivided into three horizons represented by Bolivina Xy-1, 

 Unigerina Xy-1, and CassiduUna Xy-1 and so on with the other 

 main divisions. The figures of each of these key species for the 

 different divisions may be placed in the proper position on the 

 charts and it becomes entirely a matter of discrimination of 

 characters to find the particular horizon where an unknown 

 sample should be placed. Endless variations in making charts 

 of this sort will suggest themselves to workers engaged in such 

 problems. 



With drilling wells where ranges are known, it is very simple 

 to keep up with the horizons if charts of the sections from nearby 

 wells or sections are worked out. Various horizons will be recog- 

 nized so that they may be picked up even in rotary drillings when 

 material from such horizons comes to the surface. The main use 

 is in the case of core samples or bit samples where the exact 

 position in the well is known but the problem becomes one of 

 placing the samples exactly in the section. It is a simple matter 

 with good samples to tell when passage is made from one clear 

 formation to another if the section has been worked out, but to 

 place random core or bit samples accurately means a greater 

 refinement of detailed ranges. Such work can be done accurately 

 however with sufficiently detailed material to start with by a 

 worker who can recognize valid distinctions and has keen powers 

 of discrimination. 



For the building up of sections continuous core samples are 

 by far the best. With overlapping cores it becomes possible to 

 build up continuous sections for long vertical distances. Next 

 to cores, samples carefully taken from cable tool drilled wells 

 are useful in working up sections. If the first sample out of the 

 bailer is taken, and samples are taken at frequent and regular 

 intervals, very good working sections may be built up. Rotary 

 well samples are often not worth the time spent on them as far 

 as gaining information for detailed work is concerned. After 



