368 PROF. J. W. JTTDD ON THE TERTIARY AND 



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



Silica 51-362 50-80 53-046 51-936 51-30 49-268 



Alumina 1-662 3-00 4-816 1-322 0-76 0-222 



Ferrous oxide 8-968 9-61 11-389 13-900 13-92 14-080 



Manganous oxide 0-332 1-08 0-078 0-250 0-25 0-381 



Lime 20-837 1935 19-808 19-363 20-15 20-256 



Magnesia 16-471 1506 11-576 13-850 14-85 14*812 



Soda 0-66 



Titanic acid 0-380 



Water 0-540 0-60 0-626 0200 0-21 0719 



100-172 100-16 101-339 101-201 101-44 99-738 



I. Analysis by Heddle of an altered diallagic augite of green colour, specific 

 gravity 3-329, from Hart o' Corry, Cuchvdlin Hills, Skye (Trans. Eoy. Soc. Edin. 

 vol. xxviii. (1879) p. 479). 



II. Analysis by Haughton of augite from Loch Scavaig. Skye (Dublin Quart. 

 Journ. Geol. Soc. vol. v. (1865) p. 95). 



III. Analysis by Heddle of greyish green altered diallagic augite from 

 Corry na Creech, in the CuchuUin Hills, Skye, sp. gr. 3-329 {loc. cit.). 



IV. Analysis by Heddle of brownish- green diallagic augite from Drum-na- 

 Eabn, in the Cuchullin Hills, Skye, sp. gr. 3-335. 



V. Analysis by Vom Eath of diallage from Skye : see Eammelsberg, Hand- 

 buch der Mineralchemie, 1st ed. (I860) p. 465. 



VI. Analysis by Heddle of dark-green diallagic augite from Loch Scavaig, 

 Skye, specific gravity 3-321 {loc. cit.). (Iron estimated as ferrous oxide.) 



These analyses show that the black augite, which is the most 

 common constituent of the gahhros of the Western Isles has 

 a proportion of ferrous oxide, varying from 9 to 14 per cent., while 

 that of the green variety is only about 5 per cent. The lime 

 averages 20 per cent. ; but the proportions of alumina and magnesia 

 are somewhat variable. The black varieties occur usually in crystals, 

 which are sometimes of large size ; the green varieties also form 

 similar crj'stals but sometimes occur in more or less rounded grains. 



The rhombic pyroxenes are very frequently present in these rocks, 

 but are in almost all cases subordinate to the monoclinic forms. 

 Owing to the confusion which has arisen with respect to the former 

 group of minerals, it will be necessary to discuss at some length the 

 question of the varieties which they present in the Western Isles of 

 Scotland. The minerals belonging to the group of the rhombic 

 pyroxenes which were first made known by mineralogists, and were 

 described under the names of hypersthene (paulite), bronzite, diacla- 

 site, and bastite, are all, as we shall show hereafter, in a more or 

 less altered condition — the rhombic pyroxenes being remarkably 

 subject to changes of several drfiferent kinds. But in the year 1855 

 Kengott discovered an unaltered mineral of the group, to which he 

 gave the name of enstatite *, and in 1862 Descloiseaux demonstrated 

 that it crystallizes in the rhombic system f. Shortly after this period 

 other forms of unaltered rhombic pyroxenes were discovered — proto- 

 bastite by Streng J, meteoric enstatite by Eammelberg § and Story- 



* Sitzungsb. Akad. Wien, vol. xvi. (1855) 162. ' 

 t Bull. Soc. Geol. Fr. ser. 3, vol. xxi. p. 135. 

 X Neues Jahrb. fiir Min. &c. 1862, p. 513. 

 § Monatsber. d. Akad. Berl. 1861. 



