THE FELSPARS. 
223 
showing a low alkali percentage— i.e., the presence of the two most 
basic Lime felspars. But the essential condition of obtaining any 
satisfactory result is the possibility of finding many symmetrical 
extinctions, and even then Dr. Becke, who has studied this a good 
deal, says that the division and discrimination leaves much to be 
desired, and should only be relied on in default of more certain 
methods. 
The optical method, however, is quite trustworthy and accurate 
when it can be applied to fragments obtained by cleavage and placed 
in definite directions in the field of the microscope. But this naturally 
demands a certain size in the crystals, so that manageable pieces may 
be detached. As an example, I may mention that in a north country 
dyke large clear glassy felspar crystals occur, which my friend Mr. 
Teall determined optically as Anorthite, the determination being fully 
borne out by a subsequent chemical analysis. 
Passing now to the general features, common to all the different 
species of Felspar, there is not much that is characteristic, at any rate 
microscopically. According to the rock in which they occur, they are 
found to enclose the various minerals associated with them, and, in 
addition, portions of the ground mass or glassy base, where this occurs, 
and sometimes, though rarely, the so-called water cavities— i.e., little 
drops of water shut in by the growing crystal. These inclusions are 
very frequently arranged in bands round the outline of the section of 
the crystal, showing that during the time it was forming changes of 
condition took place, and this is also strikingly shown by the Zonal 
structure, as it is called, even when there are no inclosures to 
accentuate it and make it visible without polarised light. A crystal 
showing this typically does not extinguish all over its extent in any 
one position between crossed prisms, but bands, more or less nearly 
following the outline of the grain, become dark, and on continuing the 
rotation, the extinction passes to other bands, showing, according to 
the optical method mentioned just now, a difference of composition 
from band to band. 
Some felspars are particularly beautiful minerals, on account of, 
the play of colours they exhibit. In addition to the beautiful green 
amazon stone from various localities, which is remarkable as being a 
pure Potash felspar, and yet Triclinic, we have the Aventurine or 
Sun Stone of Norway; Oligoclase, with extraordinarily delicate flakes of 
a mineral which is probably Hematite, and the well-known Labradorite 
from Paul Island, Labrador, with its exquisite play of blues and 
greens, which is also due to inclusions in its substance, though the 
nature of them is at present quite a matter of dispute. Those who 
went to Oxford the other day saw in the new schools a beautiful piece 
set in amongst the marbles of the staircase, and in the Museum is a 
fine slab of considerably larger dimensions. I am sorry that I can 
show you to-night only small specimens, but there is a case of 
beautiful polished specimens in the Corporation Art Gallery which 
will well repay a visit and examination. 
