OF THE WESTERN ISLES OF SCOTLAND. 379 



Analyses of the Later-Terfciary Augite-andesites (Pitchstone, 

 Tholeites, &c.). 



I. II. III. IV. V. VI. 



Silica 52-68 57 -57 o8-67 65-49 66-62 65-81 



Alumina 12-66 U-42 14-37 14-66 14-02 14-01 



Ferric oxide 17-34 6-04 1-64 5-73 4-43 



Ferrous oxide .3-95 6-94 .5-44 



Manganic oxide . . . trace (J-27 trace 



Lime 1145 687 7-39 3-72 2-74 2-01 



Magnesia 0-93 4-24 4-65 1-57 0*33 0*89 



Soda 2-49 2-98 3-01 6-93 4-15 



Potash 1-91 1-08 1-42 1-51 6-08 



Loss in ignition ... 0-70 1-55 2 02 2-83 2-70 



I. Analysis of augite-andesite (tholeite) with very little glass, 

 from N.W. spur of Beinn Hiant, by W. Tate. Made in the Chemical 

 Research Laboratory of the Normal School of Science, 1888. 



II. Analysis of Cleveland Dyke (tholeite), made by Stock. 



III. Analysis of crystalline portion of the Eskdalemuir dyke, bv 

 Mr. Grant Wilson, 1880. 



IV. Partial analysis of kernels of glassy rock in the Eskdalemuir 

 dyke, by Mr. Grant Wilson, 1880. 



V. Analysis of glassy andesite from west side of Ben Hiant, 

 made by Mr. T. H. Holland in the Geological Research Laboratory 

 of the Normal School of Science, 1889. 



VI. Analysis of the glassy andesite of the Sgiirr of Eigg, made by 

 Mr. Barker North in the Chemical Research Laboratory in the 

 Normal School of Science, 1888. 



These younger augite-andesites seem to be remarkable for the 

 tendency of the crystalline to separate from the glassy portions of 

 the mass. This is illustrated in the several ejections from the same 

 volcano, as in the case of Ben Hiant, and even in different parts 

 of the same dyke as shown by the interesting observations of Dr. A. 

 Geikie on the Eskdalemuir dyke, and in those of Mr. Clough in the 

 case of other dykes in Scotland. According to Professor Ditmars 

 analyses, the mass of the Dunoon Dyke contains only47'36 percent, 

 of silica, while the glassy segregation-veins contain 68-05 per cent, 

 of silica*. The contrast in chemical composition between some 

 of the Ben Hiant rocks, which are composed almost wholly of 

 crystals, and others made up almost entirely of glass, must be 

 equally great. 



The tendency of the ground-mass in rocks of this kind to ooze out 

 from among the crystals and till up vesicular cavities in the rock 

 has already been pointed out, and has been remarked upon hj 

 Osann t and Teall J. 



The dykes of pitchstone which traverse the basalt of Mull, near 

 Carsing and elsewhere, and also of the well-known dykes with the 

 same relations in Eigg, are usually glassy andesites, the porphyritic 



* Trans. Roy. Soc. Edinb. vol. xxxv. (1888) p. 44. 

 t Neues Jahrb. fur Min. Sec. 1889, vol. i. p. 304. 

 t Geol. Mag. dec. 3, vol. vi. 1889, pp. 481-483. 



