THE FELSPARS. 



the production on the surface (it is no lon<,'C'r truly ii plane) of easy 

 cleavaj^e of a number of ridijes and furrows. These are the cause of 

 the striations visible even to the uaked eye on broken surfaces of these 

 felspars which make them frequently easy to distin<^uish from Ortlio- 

 clase. In the lai'j,'e crystal in Granite on the table an included t,'rain 

 of slightly different lustre is visible, and a little further observation 

 shows the striae on it, proving that it is an inclusion of one of the 

 Plagioclases. It is, however, to the more basic rocks, such as Basalts, 

 Dolerites, and Gabbros, that we must go for the most extended 

 presence of these minerals, in contradistinction to the presence of 

 Orthoclase in the acid rocks, such as Granite, Obsidian, and Porphyry. 

 It must not, however, be for one instant supposed that the separation 

 between what we may call Orthoclase and Plagioclase rocks is a sharp 

 one. Almost all rocks which contain felspar contain a triclinic one to 

 a greater or less extent — even Granite, as meutioned above ; and ou the 

 other hand, Orthoclase is by no nieaus unknown, even in those rocks in 

 which the prevailiug felspar is triclinic. 



As to the composition of the different species which make up this 

 group, they are naturally divided into three sections : the pure Soda 

 felspar Albite, the pure Lime felspar Anorthite, and the mixed 

 felspars OUgoclase, Andesine, and Labradorite. It is still a disputed 

 point whether these last three are really definite minerals, or 

 only mixtures in various proportions of the other two. Szabo is 

 convinced, by the examination of many thousands of specimens, 

 by means of their flame I'eactious, that the series from Albite to 

 Anorthite is a perfectly coutinous one. On the other hand, other 

 observers consider that the compounds named are definite and 

 invariable, and that differences of composition are at any rate, to a 

 considerable extent, due to the interlamination of felspars of different 

 species. Dana adduces in favour of the latter view, the fact that 

 different felspars are frequently found iutercrystallised ; that in these 

 cases there is no appearance of indefinite shading of one into the 

 other, but that both keep perfectly and sharply distinct. I exhibit a 

 specimen, showing this in a striking manner. Ou the other hand, it is 

 quite certain that some of them — e.g., Oligoclase — have definite optical 

 I)roperties, and a tolerably definite composition. But we must confess 

 that variations are decidedly more common and larger than can be 

 very easily accounted for. I may perhaps give an instance — Professor 

 Heddle, in his analysis of Scotch felspars, gives one of an Oligoclase 

 from Lairg in Sutherland. I have made one of a specimen collected 

 by Professor Lapworth, in Sutherland, and find a very complete 

 accordance, except that the potash is a trifle higher and the lime 

 correspondingly lower. On cutting a thin slice parallel to the basal 

 cleavage, the reason becomes pretty certain. The greater part of the 

 mass is Oligoclase, extinguishing at the low angle from the twinning 

 plane which is characteristic of it but interlaminated with it is another 

 felspar, which by its angle of extinction is shown to be Microline. 

 Now this latter is a potash felspar, so that its presence would 



