466 



Transactions. 



a greenish tinge was noticed, due, of course, to MnOg, but the quantity pre- 

 sent was too small to be worth tabulating. The TiOo was determined 

 by the colorimetric method, but the quantity present was very minute, 

 being also not worthy of tabulation. 



W. C. Brogger, in his " Das Ganggefolge des Laurdalits," has given 

 several diagrams of rocks based on their chemical composition, or, rather, 

 what Professor Washington, in his " Quantitative Classification of Igneous 

 Rocks," calls " molecular proportions." A consideration of the above 

 analysis will show — [a) a percentage of silica which is exceptionally 

 low — even too low for a diorite, but quite usual for a gabbro : (6) a very 

 large percentage of CaO and AUOg, and at the same time an exception- 

 ally low one of potash ; this shows, as has been previously shown from 

 the extinction-angle, that the feldspar in the rock is almost wholly a nearly 

 pure anorthite : (c) a rather high percentage of total iron and a truly 

 high percentage of FcaOg. It may be remarked that a more than ordinary 

 amount of black magnetite grains were found in powdering the rock, and 

 an examination of a section shows that magnetite sometimes occurs to a 

 considerable extent. 



Professor Washington, in his " Chemical Analysis of Igneous Rocks, "^ 

 gives (p. 286) the analysis of a hornblende-gab bro which runs as follows : — 



Per Cent. 

 SiO, .. .. .. .. .. 49-80 



Al^C. 



Fe,03 



FeO 



MgO 



CaO 



Na20 



K^O 



H,0 



19-96 

 6-32 

 0-49 

 7-05 



11-33 

 2-22 

 0-61 

 1-71 



Note. — The analysis is marked A2II, so the results can be rehed on. 



Comparing this with the analysis of the rock in the claim, we see that^ 

 except for the low percentage of FeO in the former, these analyses agree 

 very well. The claim, therefore, placed as nearly accurately as possible, 

 will have the following position in Professor Washington's scheme of classi- 

 fication : " Class II. DosALANE. Rang 4. Docalcic. Hessase." 



It will be noted from the foregoing that the rock will be more accurately 

 classed as a hornblende-gabbro or simple gabbro than as a diorite, though 

 the latter name was given to it by Captain Hutton and Sir James Hector. 



Considering now this rock only as far as regards the claim, we find 

 above a series of strata of which a typical exposure is illustrated in the 

 following section :■ — 



iiominiTiTrEnTmiTnirmi; 



Bed-rock of gabbro 



I Clay 

 '—Lignite 

 . Sandstone 

 Z Lignite 



Sandstone 

 'Lignite: 



Sandstone 

 (finer) 



Fig. 8. 



