Frazer. 



100 



Dec. 20, 



In the presenl case tables of comparison were made on the basis of 2 L-f-P, 

 and 3L-| P,*but none agreed so closely as thetwofirsl made and presented 

 above. 



[t should be mentioned that a slight error is due to the counting of all 

 the titanic oxide and phosphoric oxide as pari of the silicic oxide, neg- 

 lecting at the same time to make the necessary allowance for the quanti- 

 valeniial ami atom-weight differences, but the amounl of these substances 

 was so small that the error will be entirely inappreciable. And besides, 

 even this small error will he avoided in the considerations presented below. 



The same is true of the method here followed, which is simply to com- 

 pare the ascertained percentages of the compounds instead of reducing the 

 analysis to percentage weight of the elements and striking a balance be- 

 tween the electro-negative and the electro-positive elements. This latter 

 method is much more exact but is too delicate and no better for the pur- 

 pose than the rough ami ready system here followed. A comparison of the 

 same bodies given above in their percentage values would be as follows : 



P. c. com- 

 position. 



1L. + 1 P. 



Silica (SiO,) 



Alumina (ALO.,) 



Iron Sesqui-oxide (1-Y<> >. 



Ferrous Oxide I FeO) 



Manganous Oxide (MnO). 



iVTagnesia (MgO) 



Lime i CaO) 



Soda (Na.,0) 



Potash (K.,0) 



Water (H.,0) 



51.50 

 16.95 

 0.67 

 4.17 

 0.13 

 7.4.-> 

 16.00 

 2.55 

 0.55 

 0.52 



A n aly si s o f 

 trap. 



+51.64 

 14.19 

 3.84 

 7.44 

 0.48 

 7.88 

 9.75 

 1.89 

 0.95 

 fl.95 



[t will be observed that the theoretical composition requires more Alumina 

 and Lime than are given in the analysis. The alkalies are about the same 

 in both, for there is a little less soda and a. little more potash in the rock, 

 which contains also more iron as both sesqui-oxide and protoxide. 



Manganese is too small to consider, as is also the slight difference in the 

 per cent, of Magnesia. 



The analysis thus considered tells us that the actual composition of the 

 rock, though near 1 : 1 of labradorite and pyroxene, is not quite that, being 

 slightly deficient in alumina and lime (Labradorite), while the excess of 

 the two oxide- of iron remind us that we are not to forget one of the most 

 generally distributed constituents of these traps — e. g. magnetite; though 

 really under the microscope this mineral is not at all prominent. 



The study of the microscopic section having led to the suspicion of cal- 



* I, stands for one molecule of Labradorite. P -lands lor one molecule of Py- 

 roxene. 

 t Including Ti0 2 and P 2 6 . 

 fgnition. 



