. 149. OF COMBINATIONS. 183 



plicable, if it is not limited to the forms of a single in- 

 dividual, but if it refers to all the crystalline varieties 

 known in a natural-historical species. It will be treat- 

 ed of more at large than the proposed limits of this work 

 Avould allow, in a particular work on Crystallography. 

 In general, those simple forms which are known from one 

 developed combination, become the foundation of every 

 farther developement ; and this method of proceeding is 

 also used in compound forms of a single individual, if some 

 of its forms can be identified with others, which have been 

 developed in other crystalline varieties of the same species. 



, 149. PYRAMIDAL COMBINATIONS. 



A combination of the pyramidal system is more 

 particularly said to possess a Pyramidal character, 

 if the simple forms contained in it appear with the 

 full number of their faces in their peculiar position. 



The binary combinations of this system, under the same 

 restrictions as to subordinate series as in . 145., are in 

 general 



i. P + n. P 4- n', 

 ii. P + n. (P 4- n') m , 

 iii. (P + n) m . (P + n') m/ . 



i. P + n. P + n'. 



1. Let n be = n + 1. Under these circumstances, the 

 forms will be consecutive members of the series . 101. 

 As such, they are in a diagonal position, and the edges of 

 combination which they produce, must be parallel among 

 each other, but at the same time they must be parallel also 

 to the terminal edges of the more acute pyramid, and to 

 those lines in the faces of the more obtuse one, which may 

 be drawn perpendicularly from the apices to the lateral 

 edges. Ex. P 1 (f) and P (P) in pyramidal Zircon. 

 Vol. II. Fig. 99. Inversely from the described situation 

 of the edges follow the above mentioned relations of the two 



