OLDER PERIDOTITES OF SCOTLAND. 363 



analyses as have been made of these felspars entirely support this 

 conclusion. These analyses are as follows : — 



Fels_pars of Gahhros of the Western Isles, and the Xorth of 

 Ireland. 



I. II. III. IV. 



SiUca 53-60 50-811 49-155 45-87 



Alumina 29-88 29480 29-620 34'73 



Ferric oxide 022 0252 1*152 



Lime 11-02 12690 15-309 1710 



Magnesia 0-07 0124 0-911 1-55 



Soda 4-92 3 922 2914 



Potash 0-80 0-552 0695 



Water 048 2481 0-730 



100-99 100-312 100-486 9925 



I. Is from Loch Scavaig, Skye. Analysis by Dr. Haughton, 

 Dubl. Quart. Journ. Sci. vol. v. (1865) p. 94. 



(^he iron has been calculated as ferric oxide, so as to correspond 

 with the other analyses.) 



II. Is also from Loch Scavaig, Skye. Analysis by Dr. Heddle of 

 specimen, consisting of a large crystal, bluish grey in colour, and 

 finely striated: specific gravity 2'715. Trans. Eoy. Soc. Edinb. 

 vol. xxviii. (1879) p. 253. 



III. Is from Hart o' Corry in the Cuchullin Hills, Skye. Ana- 

 lysis by Dr. Heddle of a specimen of an unstriated variety occurring 

 in granules. Trans. Roy. Soc. Edinb. vol. xxviii. (1879) p. 253. 



lY. Pelspar from the Carlingford 3tIountain, County Down : by 

 Dr. Haughton. Phil. Mag. ser. iv. vol. xix. (1860) p. 13. 



While I. agrees very fairly with the analyses of typical labrado- 

 rites, III. approaches very closely in composition to anorthite, and 

 lY. is a perfectly typical anorthite ; in II. the proportions of lime 

 and soda are intermediate between those characterizing the two 

 species. 



The presence of water, ferric oxide, and magnesia, indicates that 

 the felspar of these rocks has undergone a certain amount of altera- 

 tion, the nature and cause of which will be discussed in the sequel. 



When studied by the aid of the microscope, these felspars but 

 rarely, so far as I have observed, exhibit examples of zoned structure. 

 Occasionally, however, we find evidence that the crystals are built 

 up of intergrowths of two different species of felspar — the well- 

 known " perthite-structure "' being produced. In the great majority 

 of cases, however, the optical uniformity of the crystals, when viewed 

 by polarized light, indicates that we are dealing with a homogeneous 

 substance, with definite optical" constants, a result in harmony with 

 those obtained by Max Schuster*. 



Some of the felspars in these rocks exhibit in the most striking 

 manner the twin lamellae, which become very conspicuous when the 

 sections are viewed with the polariscope. But while certain crystals 

 * Tschermak's Min. Petr. Mitth. vol. iii. (1880) p. 117. 



