536 MESSES. HILL & KYNA8T0N ON KENTALLENITE [Aug. I9OO, 



marked degree. The rocks are especially remarkable for a high 

 proportion of orthoclase, in association with minerals such as olivine, 

 augite, and hypersthene. 



Reviewing the rocks of this class as a whole, from the areas 

 above referred to, we see that our group consists of a graduated 

 series, of which the more basic end is represented by the Glen- 

 Orchy rock, and the less basic end by the rock of An Sithein, 

 in the Ben-Bhuidhe area ; while between these extremes comes 

 the representative type of Kentallen and Loch Avich. At the 

 more basic end we have a very high proportion of olivine and 

 augite, while the proportion of orthoclase is relatively small, and 

 is exceeded by that of the plagioclase. In the intermediate mem- 

 bers of the group (Kentallen, etc.) the proportion of felspar to 

 olivine and augite is found to have slightly increased, and the 

 two felspars are present in approximately equal, though slightly 

 varying, amount. Finally, at the less basic end of the series, 

 represented by the Ben-Bhuidhe rocks, although there is still a 

 high proportion of pyroxene, that of the olivine has decidedly 

 decreased ; while there is a marked increase in the proportion of the 

 felspar, and in the An-Sithein variety orthoclase is in excess of 

 plagioclase. In fact the An-Sithein rock might almost be designated 

 a pyroxene-orthoclase-rock with olivine, biotite, and plagio- 

 clase. Thus, a progressive decrease in the amount of olivine is 

 accompanied by a corresponding increase in the proportion of 

 orthoclase. 



This less basic example of the Ben-Bhuidhe varieties will at once 

 recall the shonkinite of Messrs. W. H. Weed & L. V. Pirsson, and 

 it will be of interest in this connexion to compare briefly the two 

 rocks. Two occurrences of shonkinite were described in 1895 by 

 those authors, from Montana, the one from Square Butte in the 

 Highwood Mountains, 1 the other from Yogo Peak. 2 Occurring as part 

 of a laccolitic intrusion in sedimentary strata of Cretaceous age, 

 shonkinite forms a differentiation-product of a syenitic magma. 

 It is essentially an augite-orthoclase-rock, with smaller amounts of 

 olivine, iron- ore, biotite, and plagioclase, while apatite, and in the 

 rock of Square Butte nepheline, cancrinite, and natrolite, occur as 

 accessories. Olivine is not an essential constituent : it may or 

 may not be present, while in the Ben-Bhuidhe rocks it plays by no 

 means an unimportant part. The biotite is characteristic, and is 

 poecilitic with respect to olivine, augite, and iron-ore, being hence 

 of later formation. Augite is the most important of the ferro- 

 magnesian constituents, and together with orthoclase determines 

 the essential character of the rock : it occurs in well-formed 

 crystals, which sometimes attain a length of 1 centimetre. The 

 orthoclase is considerably in excess of the plagioclase, and occurs as 

 broad plates in which, as in the An-Sithein rock, the plagioclase- 



1 Bull. Soc. Geol. Am. vol. vi (1895) pp. 400-22. 



2 Am. Journ. Sci. ser. 3, vol. 1 (1895) pp. 467-77. 



