540 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



Class — Dosalane. Order — Germanare. 



Rang — Mon zonase. 



Subrang— Akerose (Magmatic Name). 



In view of the decomposition which this rock lias under- 

 gone, as evidenced by the presence of calcite, chlorite, and 

 other decomposition products, it is necessarily difficult to 

 form a satisfactory estimate of the " mode," and it is still 

 more difficult to satisfactorily calculate the mode of the 

 unaltered rock. The calculation is given in Appendix I 

 with the following result : — 



Ilmenite 2*2 



Orthoclase 



Albite... 



Anorthite 



Magnetite 



Pyroxene 



Apatite 



18-2 



56*3 



4-0 



2*8 



15*8 



0'7 



100*0 



The amount of pyroxene shown is somewhat larger than 

 the examination of a number of slides led one to expect. 



Appendix I. 



Determination of the Mode. 

 The calculation of the mode, or actual mineral compo- 

 sition, of a rock is possible, as pointed out by Cross, Iddings, 

 Pirsson and Washington, 1 only when it contains not more 

 than one ferric or alferric mineral, unless the compositions 

 of any such minerals present be known. 



If there be more than one alferric mineral present, it is 

 necessary to know, not only the composition of each, but 

 also the quantity present of all but one. Hence the calcu- 

 lation of the mode entails, if it is to be done accurately, 



1 Quantitative Classification, p. 210. 



