578 



CROSS, IDDINGS, PIRSSON, WASHINGTON 



It will be noted that the rocks, or mineral or chemical groups, 

 corresponding to the center-points of these divisions seem to 

 have a special value as classificatory types, but we wish to point 

 out that this is not actually the case. Such rocks or such 

 groups, are not more important, considered quantitatively, than 

 those occurring in any other part of the system, even on the 

 boundary between neighboring divisions. 



This fivefold method of subdivision may be expressed graph- 

 ically as follows : 



B 



These divisions or units may be described as : 



I. One in which the center-point is where the first group is present alone, 

 8:o. 

 II. One in which the center-point is where the first group is three times the 

 second, 6 :2. 



III. One in which the center-point is where both groups are present in equal 

 amounts, 4 : 4. 



IV. One in which the center-point is where the second group is three times 

 the first, 2 : 6. 



V. One in which the center-point is where the second group alone is present, 

 0:8. 



The dividing lines between these center-points will occur at 

 the following ratios : 7:1,5:3,3:5, and 1 : 7. The ranges of 

 the five divisions are given by the expressions : 



d)i>\, (")i<f>f . < iii )|<f>i- 



( IV >!<f>7> CV)|<i. 



The divisions may be defined in general terms as : 



