Vol. 56.] AND OTHER IGNEOUS ROCKS IN ARGYLLSHIRE. 539 



gabbro-diabase, 1 and now included in his monzonite-group. The 

 rock contains accessory olivine and a small proportion of orthoclase. 



Moreover, the occurrence of a considerable proportion of a rhombic 

 pyroxene in addition to augite in the Allt-an-Sithein rock recalls 

 certain hyperites and norites, in which orthoclase frequently 

 occurs in addition to plagioclase, but without the association of 

 olivine. Such are the hyperites, described by Mr. Teall, 2 asso- 

 ciated with the Loch Dee granite of the Southern Uplands. These 

 rocks show affinities to the so-called norites of Teller and Von 

 John in the neighbourhood of Klausen in the Tyrol, and are essenti- 

 ally composed of plagioclase, hypersthene, augite, and biotite, with 

 small quantities of quartz and orthoclase occurring as interstitial 

 matter. Among the Cortlandt Series of the Hudson River the norite 

 proper, described by G. H. Williams, 3 consists mainly of andesine and 

 hypersthene, with accessory biotite ; and an interesting feature is the 

 occurrence of large crystals of orthoclase enclosing the other minerals 

 in a poecilitic manner : the hypersthene may be associated with 

 augite, and sometimes hornblende. Then, again, in the Lake- 

 Superior region we have the ; orthoclase-gabbro ' of Irving, 4 in which 

 the plagioclase is oligoclase or an allied variety, and orthoclase 

 occurs in addition. 



It is interesting to note that in many of these basic rocks which 

 are characterized by the occurrence of orthoclase, the plagioclase- 

 felspar belongs rather to the more acid series than to the more basic ; 

 that is to say, one frequently finds felspars of the oligoclase-andesine 

 variety rather than those of the labradorite-anorthite series, which 

 one would more naturally expect in rocks of thoroughly basic 

 character. Thus in the shonkinite of Yogo Peak the plagioclase is 

 andesine ; in the olivine-monzonite of Smalingen it is andesine as 

 well as labradorite. We have andesine again in the norite of the 

 Cortlandt Series, and oligoclase, or an allied variety, in the ' ortho- 

 clase-gabbro ' of the Lake-Superior region ; while in kentallenite the 

 plagioclase ' probably ranges in composition from labradorite to 

 oligoclase, the acid type predominating.' We may further note in 

 this connection that in kentallenite the plagioclase, orthoclase, and 

 biotite, together play the part of groundmass to the olivine and 

 augite, a feature especially well brought out in the variety of the 

 Glen-Orchy rock already described (p. 534). (We thus have an 

 earlier generation consisting essentially of olivine and augite, and 

 a later consisting of a more or less acid plagioclase, orthoclase, 

 and biotite. The chemical composition of this groundmass would 

 probably approximate to that of a mica-diorite, when plagioclase 

 was in excess of orthoclase, and would probably resemble closely 

 that of many of the relatively more acid rocks of the same district, 

 while the earlier-formed constituents would be of ultrabasic com- 



1 Brogger, ' The EruptWe Eocks of Gran ' Quart. Journ. Geol. Soc. vol. 1 

 < 1894) p. 19. 



2 Ann. Rep. Geol. Surv. 1896 [1897] pp. 41 & 42. 



3 Am. Journ. Sci. ser. 3, vol. xxxiii (1887) pp. 135-44, 191-94. 

 * U.S. Geol. Surv. Monogr. no. 5 (1883) pp. 50-56. 



