106 



DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



ing an average proportion of the calcitic lenses was again felt here, as it was 

 in selecting material for the analysis of the Siyeh rock of the Lewis series. 

 It was found that in spite of all care different powders intended to represent 

 the average gave different analytical results. Two total analyses of such 

 powders (A and B) were made by Professor Dittrich, who also determined the 

 portions of A soluble and insoluble in acid. The mean of A and B, shown in 

 the following table, is doubtless the average chemical composition of the rock 

 more nearly than either A or B. 



Analyses of type specimen, Siyeh formation. 



A. 



Si0 2 36-64 



Fe 2 3 .99 



B. 



36-97 

 7-59 

 1-82 

 1-12 

 8-38 



16-28 



1-04 



2-48 



■24 



311 



•08 



21-11 



Mean of 

 A and B. 



36-80 



5-92 



1-40 



•85 



6-38 



21-03 



•76 



1-68 



•23 



2-49 



•08 



22-71 



Molec. prop 

 in mean. 



•613 



•058 

 •009 



FeO -57 



MgO 4-38 



CaO 25-79 



Na„0 -49 



K 2 -88 



ELO at 110°C -22 



•012 

 •150 

 •376 

 •012 

 •018 



H,0 above 100°C 1-87 



•139 



C 



C0 2 24-31 



•516 



100-38 100-22 

 Sp.gr 2-748 



Portion of ' A ' soluble in hydrochloric acid : 



Fe„0, 



100-33 



42-46% 

 1-33 



Al„0 3 







•44 



CaO 







25-52 



MgO 







3-75 



Practically all the lime is soluble in acid and may be directly assigned to 

 the carbonate. It is probable that the soluble magnesia of the average rock 

 occurs in nearly the same proportion as in A, so that of the 6-38 per cent of 

 the mean credited to magnesia about 5-5 per cent should be regarded as assign- 

 able to magnesium carbonate. Arbitrarily assigning the soda to the albite 

 molecule, the potash to the orthoclase molecule, and the iron oxides to magne- 

 tite, the ' norm ' of the rock may be calculated in the same way as it was for 

 the Siyeh rock of the Lewis series. The result gives col. 1 of the following 

 table : — 



Calcium carbonate.. . 

 Magnesium carbonate. 

 Silica 



Orthoclase molecule. 

 Albite molecule. . .. 



Magnetite 



Remainder 



1. 



2. 



37-6 



35-3 



11-6 



18-6 



26-0 



28-0 



10-0 



7-2 



6-3 



4-2 



2-3 



2-6 



6-2 



3-9 



100-0 



100-0 



