70 C. A. SUSSMILCH. 



It would thus appear that masses of holocrystalline basic 

 rocks must exist at no great depth in this portion of 

 Australia." Mr. Card informs me that in examining many 

 of the other dyke rocks he has noticed many inclusions of 

 foreign crystals (Xenocrysts) similar to those already quoted. 



The occurrence of the fragments of gabbro and peridotite 

 in the dyke at Kiama points to a similar conclusion to that 

 arrived at by Mr. Card, and considerably extends the area 

 beneath which basic and ultrabasic plutonic rocks probably 

 exist in eastern New South Wales. 



NOTE ON SOME SIMPLE MODELS FOR USE IN 



THE TEACHING OF ELEMENTARY 



CRYSTALLOGRAPHY. 



By W. G. WOOLNOUGH, D.Sc, F.G.S. 



(Communicated by Prof. T. W. E. David, b.a., f.r.s.) 



[Read before the Royal Society of N. 8. Wales, October 4, 1905, ,] 



In the course of nearly ten years' experience in the teach- 

 ing of elementary crystallography, I have found it very 

 difficult to make the average student appreciate the con- 

 nection between the number of faces in a crystallographic 

 "form" and the elements of symmetry characteristic of 

 the group to which the crystal belongs. I have, therefore, 

 prepared ,several very simple models which, I find, make 

 the understanding of this very important point perfectly 

 easy to everyone. 



A plane of symmetry divides a crystal into two portions 

 which are to one another as an object and its reflection in 



