220 
THE FELSPARS. 
the production on the surface (it is no longer truly a plane) of easy 
cleavage of a number of ridges and furrows. These are the cause of 
the striations visible even to the naked eye on broken surfaces of these 
felspars which make them frequently easy to distinguish from Ortho- 
clase. In the large crystal in Granite on the table an included grain 
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 mentioned above ; and on the 
other hand, Orthoclase is by no means unknown, even in those rocks in 
which the prevailing 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 Oligoclase, Andesin, and Lahradorite. 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 reactions, that the series from Albite to 
Anorthite is a perfectly continous 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 intercrystallised ; 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. On the other hand, it is 
quite certain that some of them— e.g., Oligoclase—have definite optical 
properties, 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 
