MICROSCOPIC STRUCTURE OP FELSPARS. 481 



milk-white felspar from Twedestrand, in Norway. The principal 

 cleavage of the specimen is marked by beautifully regular and 

 parallel striations, visible to the naked eye ; and to all appearance 

 the felspar is a triclinic one, in outward aspect resembling oKgoclase. 

 In thin section under the microscope, when seen by polarized light, 

 it displays in different positions of the Nicols even sheets of colour 

 alternately pale blue and pale orange, and shows, in places, broad 

 bands, feeble in tint but with well-defined margins. Scattered 

 throughout the mass are little irregularly shaped patches which 

 show the cross-hatched striation most distinctly. These patches 

 are, no doubt, orthoclase*. Their striation invariably corresponds in 

 one of its directions with the broader striation of the surrounding 

 mass. Having ascertained this point, it remained to be shown 

 whether any of these patches exhibited partial striation ; for here no 

 valid objection could be raised on the score of the sectional planes 

 not being the same, as, although the markings in some of the isolated 

 patches became more faint in one direction, similar markings in 

 other patches became fainter in a direction exactly opposite, as 

 shown in the accompanying drawing. If, therefore, widely distant 

 patches in the section exhibited the cross-hatching over their entire 

 surfaces, it was obvious that if the cross-hatching were partially 

 absent in any patch the defect could not be due to any deviation of 

 the plane of section. Several patches occur in this section in which 

 the cross-hatching is only partially displayed. One such patch is 

 shown in fig. 6, Plate XXIII., under a magnifying power of 350 dia- 

 meters. A section of oligoclase from the zircon syenite of Hamilton 

 Sound, Labrador, collected by my friend Mr. Bauerman, exhibits in- 

 cluded patches of cross-hatched orthoclase similar to those just 

 described. It appears needless to multiply instances of partial 

 failure in the development of this structure. It is sufficient for me 

 to demonstrate the facts ; theories concerning their cause I leave to 

 other and more skilled observers. The next thing to do was to find 

 cross-hatched and unstriated or simply striated matter symmetrically 

 •distributed in one and the same individual. In fig. 3, Plate XXIII., 

 we meet with such an example in a twinned sanidine crystal in 

 trachyte, from Berkum, on the Rhine. This section is magnified 

 115 diameters (the same power as that employed in the delinea- 

 tion of fig. 5). We here notice that the striations transverse to 

 the longer diameter of the crystal are much more delicate than in 

 fig. 5, that they are crossed at right angles by bands some of which 

 are apparently coarser, and that they occupy well-defined areas 

 which are segments either of circles or ellipses on either side of 

 the crystal, while the terminal spaces are unstriated or marked only 

 by very feeble striae, which assume directions approximating to those 

 of the two arcs which form the large internal boundaries. 



We now come to a remarkable form, which may serve in some 

 measure to elucidate points of structure in the foregoing example. 



* Allusion was here made to the observations on the intergrowths or admix- 

 ture of different felspars recorded by Tschermak in his well-known paper 

 Sitzungsberichte d. kais. Akad. Wiss. Wien, Bd. i. Abth. i. 571 (1864). 



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