10 THE FELSPARS. 



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 Trirlinic, we have the Aventurine or 

 Sun Stone of Norway ; Oligoclase, with extraordinarily delicate ilakes 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. 



