Vol. 56.] AND OTHER IGNEOUS ROCKS IN ARGYLLSHIRE. 537 



laths lie scattered about unorientated. These broad areas of ortho- 

 clase not only contain the plagioclase, but also the other constituents 

 in a pcecilitic manner. 



In its chemical characters shonkinite is generally related to 

 rocks of the basic class, low in silica, and high in magnesia, lime r 

 and iron-oxide, and related to rocks of the lamprophyre-family. It 

 stands very close chemically, as Messrs. Weed & Pirsson remark, to 

 certain vosgesites and minettes, while differing from them in 

 mineral composition and structure. It bears the same relation to 

 augite-syenite that minette does to mica-syenite. The resemblance, 

 therefore, between shonkinite and the rock of Allt-an-Sithein is still 

 further of interest, in view of the evidence of the association in the 

 Ben-Bhuidhe area of dykes and sills of lamprophyre with the kent- 

 allenite intrusions. These considerations, and the fact that the 

 An-Sithein rock shows a marked variation from the more normal 

 (Kentallen) type of the group, would naturally lead one to suppose 

 that the chemical composition of the Ben-Bhuidhe rock would still 

 further demonstrate on the one hand its affinities to such rocks as 

 shonkinite, and on the other hand the points in which it differs 

 from the Kentallen rock. A complete analysis was kindly prepared 

 for us by Dr. W. Pollard, of the Geological Survey, with the 

 following result : — 



Ti0 . 

 Al-A . 

 0r 2 O 3 . 

 Fe0 3 . 

 FeO . 

 MnO . 



^er cent. 



52-09^ 



•73 



11-93 



•10 



1-84 



711 



15 



(CoNi)O "° 7 I -10024 



CaO 7-84 f-WU^ 



MgO i 12-48 



K 2 301 



Na 2 204 



CO; -16 



PA 34 



Cl trace 



H 2 G -35; 



Specific gravity =2"94 



The result of the foregoing analysis fully bears out the obser- 

 vations made from the microscopic examination of the rock-section. 

 The presence of chromic oxide probably indicates the occurrence of 

 a small quantity of chromite or picotite, which is known to be of 

 common occurrence in rocks rich in olivine, though it was not 

 detected with certainty under the microscope. For the purposes of 

 comparison, it will be sufficient to consider only the principal rock- 

 forming oxides ; and these we will compare with the published 

 results of the analyses of the Kentallen rock, shonkinite, and the 

 olivine-monzonite of Prof. Brogger, from Smalingen, in Sweden, 

 We thus have the following table : — 



