266 



FRANK D. ADAMS 



Albite 



Anorthite . . . 

 Orthoclase . . 

 Nepheline. . . 



Kaolin 



Pyroxene .... 

 Hornblende . 



Biotite 



Olivine 



Magnetite. . . 

 Ilmenite .... 



Apatite 



Water (hygr.) 



Essexite 



(Analysis i) 



Mount Johnson 



36.75 



20 



9 

 3 



5.68^ 



66.45 



4-77 

 6.29 



7-05 

 2.04 

 none 



9-53 



2.68 



.58 



99-39 



Olivine-Es.siexite 



(Analysis 2) 

 Mount Johnson 



3-94 \ 



4-47 S 



54-79 



II .90 



12.22 

 2.30 

 4.08 

 2.84 



8.41 



2.59 

 ■«5 



99.98 



In the case of No. i the percentage mineralogical compo- 

 sition given expresses exactly the chemical composition of 

 the rock, except that it requires 0.06 per cent, of Fe O in 

 excess of that shown in the analysis. In No. 2 the agreement 

 is complete. 



The calculation further demonstrates that the plagioclase in 

 the case of No. i is a trifle more basic, and in the case of No. 2 

 a little more acid, than Ab^ An^, which as has been stated, is 

 shown by the optical character and by the specific gravity of the 

 feldspar to represent its average composition in these rocks. 

 The amount of orthoclase recognized in thin sections also 

 appears as mentioned in the description of the rock. The 

 nepheline is in places somewhat altered to a mineral resembling 

 kaolin. The small percentage of kaolin shown by the calcula- 

 tion has therefore been added to the nepheline in extending the 

 table. 



In order to fix the position of these rocks m the excellent 

 system of classification recently elaborated by Messrs. Cross, 

 Iddings, Pirsson, and Washington, and to determine the name 

 which should be given to these rocks, if their precise character 

 is to be designated, it is necessary to calculate their ?iorms. 

 These have been found to be as follows : 



