8o 



ALBERT JOHANNSEN 



feldspathoid rocks are similar to those in Fig. 15, but are somewhat 

 more irregular owing to insufficient data. 



The families are to be numbered as shown in Fig. 16. The 

 object in beginning with o is to make the positions easier to remem- 

 ber, since they run in groups of five. Furthermore, Family o 

 occurs only in Order 1, as do also Families 1, 6, 11, 16, 21, 26, and 

 31, for they form the hinge about which the order tetrahedron 



(Fig. 10) was opened, and are 

 the same in all. This is shown 

 in Figs. 21 to 23, where these 

 families are omitted and repre- 

 sented by dotted lines. Instead 

 of having 12X32 families, there- 

 fore, there are 3X32 families (in 

 the first orders in each of the 

 -r| Plag f rs t three classes) +9X 24 fami- 

 lies (in Orders 2, 3, and 4) +3 

 X15+1 families (in Class 4, to 

 be mentioned later), making 

 358 families in all. If Order 1 

 is omitted, as suggested in ques- 

 tion 4, below, the total families 

 will be 286, and if Order 4 is 

 united with Order 3 there will 

 be only 214. Although the 

 maximum number of families is 

 358, it does not mean that there are 358 names to learn, for the 

 light and dark rocks may be separated by prefixes without making 

 awkward names; thus leuco-granite, melano-granite, etc. 



The divisions made by other writers may now be compared 

 with Figs. 14 and 15. Lincoln uses the ratio orthoclase to all 

 plagioclase, the latter not differentiated as is done here. His per- 

 centages are 100-96-67-33-4-0. 



It is rather difficult to compare the divisions proposed by Iddings 

 with those proposed by Lincoln or by the present writer, for, as 

 mentioned above, he unites albite with the potash feldspar and 



Folds 

 Fig. 16. — Family numbers in Classes 

 1 to 3. 



