PETROLOGICAL ABSTRACTS AND REVIEWS .485 



MgO, and c for the remaining CaO. If all of the CaO was used for feldspar, 

 the excess of AI2O3 is represented by t among the mafic constituents. If there 

 is insufficient AI2O3 to satisfy the alkalies, the remaining NazO is indicated by 

 n and it also is placed among the ferromagnesian constituents. In the above 

 example, K20+Na20 = 4.i+3.3 = 7.4, corresponding to 7.4 AI2O3 leaving 1.2 

 AI2O3 to unite with CaO for the anorthite radical. The remaining 0.2 CaO is 

 represented by c among the mafic constituents. The ferromagnesian con- 

 stituents are given in the order of decreasing abundance, and their sum is 

 represented by the figure following (here 2.6). The relative abundance is 

 indicated in the symbol, thus 



fm indicates an FeO/MgO ratio below % FeO/4 MgO. 



f = i| f, therefore fm shows a ratio between 4FeO:3MgO and 3FeO: 2 MgO. 



f=2 f, or fm indicates the ratio of 2/1. 



F = 3f, and Fm = 3 FeO to i MgO. 



F = 4f, and Fm = 4/i. 



F=5f, and Fm=5/i. 



F2=6f, 



F. = 8f, 



F3=iof, 



F4= i2f, etc., etc. 



Since the remaining constituents in the order of decreasing abundance in the 

 example given are FeO=i.5, FczOj^cs, MgO = o.4, and CaO = o.2 (the ratios 



being therefore as 7 : 2.5 : 2 : i approximately), they are represented by F2 : e : 



m : c. (These symbols are complicated. Why not use the ratio directly as 

 subscripts; thus here F1.5 . Co-s . mo.4 . C0.2, or even the simplified similar ratios 

 F7 . e2.s . m2 . c). 



The rock may be plotted on rectangular co-ordinates as shown by the figure. 

 The horizontal line {S axis) represents the silica content. The vertical line 

 through 50 per cent is the S axis and is the measure of the feldspar content. 

 It is drawn at double the scale of the S axis because for each molecule of alkali 

 or lime, a molecule of AI2O3 is also automatically included. Further, there are 

 two diagonal lines at 45° through the 50 per cent and 100 per cent points on the 

 5 line; the former is called the quartz normal, the latter the F axis. 



The Brocken granite is thus plotted by measuring from the 80.2 point on the 

 5 axis downward 4.1 for K2O. This K point is indicated by an x. A further 

 distance of ^.2, for Na20 is laid off on the same line and the point is marked by a 

 circle. This is called the Locus of the rock {Ort des Gesteins). Finally 1.2 is 



