538 MESSRS. HILL & KYTNASTON ON KENTALLENITE [Aug. I9OO, 



I. II. III. IV. V. 



Per cent. Per cent. Per cent. Per cent. Per cent. 



Si0 o 52-09 48-00 48-49 4673 50-35 



Al 2 a 11-93 12-52 12-29 1005 1576 



} 8-95 8-74 8-65 1173 



MgO 12-48 15-26 9-91 9"68 7*40 



OaO 784 794 965 1322 1012 



Na.,0 204 311 222 181 275 



K 2 301 2-68 4-96 376 389 



Specific gravity ... 2'94 2"95 — — — 



I. Kentallenite, Allt-an-Sithein, Glen Shira, Argyll. (Analysis by Dr. 



Pollard.) See PI. XXX, fig. 2. 

 II. Kentallenite, Kentallen, near Ballachulish, Argyll. (J. J. H. Teall, Ann. 

 Kep. Geol. Surv. 1896 [1897] p. 22.) 

 III. Shonkinite, Yogo Peak (Montana), U.S.A. (Weed & Pirsson, Am. Journ. 



Sci. ser. 3, vol. 1, 1895, p. 478.) 

 IV. Shonkinite, Sqnare Butte, Highwood Mountains (Montana), U.S.A. 



(Weed & Pirsson, Bull. Geol. Soc. Am. vol. vi, 1895, p. 414.) 

 V. Olivine-monzonite, Smalingen, Sweden. (Brogger, ' Die Eruptionsfolge 

 der triadiscken Eruptivgesteine bei Predazzo in Siidtyrol' p. 50.) 



It will be seen that, with the exception of the silica and magnesia, 

 there is a striking correspondence between the Allt-an-Sithein rock, 

 that from Kentallen, and the shonkinite of Yogo Peak. The Allt- 

 an-Sithein rock, being a slightly less basic variety, yields a higher 

 silica- and lower magnesia-percentage than that of Kentallen, the 

 microscope showing that it is richer in orthoclase and poorer in 

 olivine. With the increase of orthoclase the relative proportion 

 of the alkalis becomes reversed, the potash exceeding the soda, 

 and this is also the case in both types of shonkinite and in 

 olivine-monzonite. Both the Argyllshire rocks contain a con- 

 siderably higher percentage of magnesia than shonkinite or olivine- 

 monzonite, while the latter two are richer in lime, presumably 

 owing to their relatively 'higher proportion of augite. Olivine 

 cannot be considered more than an accessory in shonkinite and 

 monzonite, while in kentallenite it is an essential constituent. 

 The analysis fully bears out the petrological evidence that the 

 rocks of Allt-an-Sithein and Kentallen belong to the same group, — 

 a group of rocks with essential olivine and augite and smaller 

 amounts of orthoclase and plagioclase in varying relative pro- 

 portions. And these rocks show affinities (1) to the picrites, in 

 their high proportion of augite and olivine ; (2) to the shonkinites of 

 America, in the peculiar association of those basic minerals with 

 orthoclase-felspar : and (3) to the monzonites of Prof. Brogger, 

 more especially to the basic variety of Smalingen, in the fact that 

 orthoclase and plagioclase occasionally occur in approximately equal 

 proportions, as in the rock of Kentallen. 



The kentallenites of Argyllshire constitute another example, and 

 one of the few known in Britain, of the peculiar association of ortho- 

 clase with olivine and augite. Besides the foreign examples of a 

 similar association already referred to, we may cite that of the rock 

 of Dignas, in Norway, described by Brogger as a variety of olivine- 



