162 Dr. J. S. Hyland—Soda-Microcline, Kilimandsci.aro. 
prepared.’ This conclusion is further supported by mechanically 
removing the outer portions of the crystals till the twin junction 
(Zwillingsnaht) is observed. In fact, we have probably here to 
deal with a nucleus twinned after the Carlsbad type, surrounded 
by felspar-substance free from such a structure. 
From a crystallographical standpoint it is interesting to note the 
angle between P and M. This I have determined on several 
cleavage pieces, and obtain as mean 90° 3’. The physical condition 
of the surfaces was not very favourable for such a determination, 
still the variation was always very small. Miers observed 90°, and 
mentions that the variation never exceeded 16’. It is interesting to 
note, that Brogger determined 90° 38’ on the felspar from the dykes 
at Fredriksvarn.’ 
On examination under the microscope Mr. Miers® observed that 
parts of the crystals viewed by polarized light exhibited a well- 
defined cross-hatching parallel to the pinacoidal faces, but could, 
however, discover no trace of striation on sections parallel to OP. 
On the contrary, all the crystals I examined—ten in number—were 
seen to be built up of fine triclinic lamelle of variable breadth. 
The examination of numerous sections cut parallel to OP showed that 
the lamellae mostly follow the albite law; in such sections the 
extinction is nearly parallel and perpendicular to the edge P:M. 
The extinction of the very fine striz on OP may be measured as 
1°—2°, the larger ones ascending to 34°. The surface of such a 
striated section under crossed Nicols is therefore never dark. 
Certain portions however exhibit no striz, in fact the lamelle 
appear to taper off and finally disappear. Brogger has made a 
similar observation. 
Sections parallel to M pesbeke an extinction-angle of 5°—6° and 
exhibit no striation. 
One very thin section parallel to OP from the middle of a crystal 
showed at certain portions, in addition to the striz of the albite 
type, a second system crossing these at right angles, thus pro- 
ducing a structure similar to though not perhaps identical with 
that of microcline. This microcline-structure is, however, to be 
seen best in sections cut at right angles to P and M. Such sections 
appear composed of very fine and distinct lamellz crossed by 
another series perpendicular to the first and of irregular breadth. 
The extinction-angle is here 8°—11° to their length, but others 
appear to possess a straight extinction. 
The triclinic character of the felspar is well marked in the rock- 
sections. The felspars of the ground-mass are identical with their 
porphyritic representatives, although the former are mostly lath- 
shaped. Where, however, the felspar forms laths of very small 
breadth, there is certainly no striation to be recognized; but this is 
an observation already made by those writers who have examined 
the felspar of the “Rhombenporphyr,” to which, as has been 
1 The sections were made by Fuess of Berlin, and Voigt and Hochgesang of 
Gottingen. 
2 Brogger, op. cit. p. 259. Silke ypamlele 
