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



As to what the composition of the rock may be at greater 

 depths we have no hint. 



The distribution of the iron ores is peculiar and calls for 

 a few special words. In considering the iron ore contents 

 of the pallio-essexite, however, account must be taken of 

 the possibility of error in the ferrous and ferric iron deter- 

 mination of the analysis of specimen B referred to in 

 Appendix I. By calculation from analysis the percentage 

 of iron ores in that specimen is only 3*7, but as obtained 

 by measurement on the Rosiwal method, and after correc- 

 tion for decomposition, it is 6*5. Repeated experience has 

 shown that the Rosiwal method is very fairly accurate, and 

 the slight tendency to exaggerate the colored constituents 

 could not account for this discrepancy. Calculation gives 

 ilmenite 3*2 and magnetite only 0*5, whereas in all the 

 other Prospect rocks for which analyses are available these 

 two minerals are present in nearly equal proportions ; and 

 it is precisely the quantity of magnetite which would be 

 affected by an underestimation of ferric iron. Hence it 

 seemed preferable to rely upon the Rosiwal method for the 

 estimation of total iron ores, and to obtain the magnetite 

 by difference between this and the quantity of ilmenite. 

 In the following table are shown the percentages of iron 

 ores in the same specimens as are represented in the last 

 table, and also the ratio of iron ores to augite in the same 

 specimens. 



I. II. III. IV. V. VI. VII. 





L. 



5 cm. 



B. 



30 cm. 



c. 



450 cm. 



p. 



500 cm. 



Survey 

 Specimen 

 >400 cm. 

 <900 cm. 



Coarse 

 Aplite 



1500 cm. 



i. 



1800 cm. 



Ilmenite . . . 

 Magnetite... 

 Total ores ... 



Ratio 



7.4 



.22 



3.2 

 3.3 



6.5 



.20 



5.6 

 .16 



li'.o 



.50 



4.8 



6.8 



11.6 



.56 



2.2 

 2.8 

 5.0 



,31 



8.4 



7.6 

 16.0 



.42 



augite 



