518 Analysis of Anorthite from a Diorite from the Oural. 



A sheet of paper being fastened on a horizontal table, ABC 

 is laid upon the table, and two signals being placed in a plane 

 through the ci-ystal parallel to the table, the crystal is adjusted, 

 by means of the rotations round EF and HG, so that the 

 edge in which the planes of two of its faces intersect may be 

 made perpendicular to the plane ABC. ABC being turned in 

 its own plane till the image of the bright signal in one face of 

 the crystal coincides with the faint signal seen by direct vision, 

 a line is drawn on the paper along the edge AB. The same 

 process is gone through with the reflected image of the bright 

 signal in the second face of the crystal, taking care that the in- 

 tersection of the faces shall occupy as nearly as possible the 

 same position during each observation, and a line is traced upon 

 the paper along the edge AB. The angle between these two 

 lines is the angle between normals to the two faces of the cry- 

 stal. This angle may be measured by describing a circle round 

 their intersection as a centre, and comparing the arc intercepted 

 between the lines, with an arc of 60°, by the method given in 

 Legendre's Geometrie, livre ii.; probleme xviii. Or, a circle 

 may be described passing through the intersection of the straight 

 lines, and the arc which they intercept compared with the whole 

 circumference by the same method. The value of the angle in 

 degrees is obtained by multiplying the ratio by 60, in the former 

 case, and by 180 in the latter. 



LXIV. Analysis of Anorthite from a Diorite from the Oural 

 Mountains. By Robert H. Scott, Esq.* 



PROFESSOR G. ROSE having requested me to undertake 

 the analysis of the felspar of a Diorite which forms the 

 Kouschekowskoi Kamm near Bogoslowsk, in the northern Oural 

 Mountains, I accordingly did so, and found it to be anorthite, 

 the results of my analysis being, — 



17-6338 

 4-9632 



100-806 



* Comiminicated by the Author. 



