116 A. W. Jackson — Nomenclature of Crystalline Bocks. 



tain fixed points in this vague and indefinite mass of tra 

 forms; or* to express myself petrographieally, in the formless 

 m gin to appear, about 

 miogeneous material begins to deposit itself concentri- 

 cally with gradually decreasing density as we recede from the 

 center. Thus arise rock-groups whose central types are .vcll- 

 deflned and distinctly separated from one another, while their 

 s tangent and in many ways coalesce. One now re- 

 turns to the definite system which no longer possesses however 

 the rigidity and dead immobility of the first period." 



,vho believes there is no such thing as a definite rock- 



type, finds himself in the second stage of his develop) 

 only needs a more extended range of experience to discover tJae 

 s and to view them in their true relations; and he 



significance of this 

 adaptability and will recognize the importance of conceding to 

 it a high degree of influence in the construction of rock-names. 



Mr. Darwin has demonstrated the necessity of recognizing 

 this principle in the organic natural sciences and the mineral- 

 in mind. 



Were the necessity at all incumbent upon us to form a system 

 of nomenclature for material that had reached a 'perfect natural 

 classification, I should certainly add a fourth principle to the 

 three thus suggested, namely, that a name should he construct- 

 ed so as to suggest the relation of the rock to all allied rocks: 

 in other words I would seek to construct a complete systematic 

 nomenclature that should be an exact expression of the system- 

 ification. 



As such a system has not yet and may never be reached, it 

 is certainly unwise to allow this principle to enter largely into 

 the determination of present nomenclature. It would at once 

 conflict with the first two principles of uniformity and stability. 

 To a slight extent it i, « done ; we shall con- 



sider such cases later. 



Having thus laid down certain principles to guide us, I will 



.econl with these principles, ihev will be endorsed: in so far 

 hev are m eontiic!. thev will bo 'rejected. 

 [claim, first, as o\ fonda hat furls, and fa. 



uld be entirely ignored. Facts are susceptible of 

 and a nomenclature founded upon them is sure 



nd stable, win < >p< u ] axon*, hypotheses and 



