Simpler Methods in Crystallography. 403 



XXVII. Simpler Methods in Crystallography. Part. IT. (1) 

 Maps of Anorthic Crystals; (2) The Determination 

 of Crystalline Forms ; (3) Methods of Drawing Crystals. 

 By J. G. GooDCHiLD, H.M. Geol. Survey, F.G.S., 

 F.Z.S. (Continued from p. 359 of the present volume.) 



(Read 17th April 1901.) 



The projection of stereograms of crystals belonging to the 

 Anorthic System involves much more complicated methods 

 of work than do any of those described in the former paper. 

 But though the principle of construction by proportional 

 parts, as applied to the other systems, is equally applicable 

 to this if these principles are rightly understood, it may be 

 as well to describe my own methods of work in making 

 maps relating to this system of crystals. 



In the other systems the poles projected to the surface of 

 the sphere as normals to each of the faces group themselves 

 about certain planes with the particular grade of symmetry 

 proper to each system. In the Anorthic System, on the 

 other hand, no such symmetry is traceable, and much of the 

 trouble involved in making these maps arises from this 

 cause, or, from what amounts to the same thing, to the fact 

 that the crystallographic axes and the axes of reference are 

 in no case the same. 



In drawing these stereograms, it has, of course, to 

 be borne in mind that the poles and their zones, sup- 

 posed to be drawn on to the sphere of projection, may be 

 viewed in any direction. We may suppose, for instance, 

 that we look at the inner surface of the sphere from the 

 opposite outer surface at c, or at &, or at a, just as occasion 

 requires, and in like manner we are at liberty to project on 

 to the plane of projection from any one point on the 

 primitive to any other, as may be needed in each case. 



I have made a large number of projections of anorthic 

 crystals, chiefly with the object of interpreting the morpho- 

 logy of actual crystals, but partly also with the object of 

 selecting one species which may serve as a typical example 

 of this system. Perhaps, all things considered, the best for 

 the purpose is Ehodonite (Dana's number 335), a manganese 



