A. W. Jackson — Nomenclatiire of Ory»t"lll,t> Boris. 115 



be no objection to the change; uniformity would still be pre- 

 served. This principle as well as the first has been frequently 

 violated in petrography. 



III. Adaptuhiiity. in considering this print- pie it is desirable 

 to bear in mind the peculiar nature of a rock us distinguished 

 from a mineral. A mineral is a definite homogeneous chemical 

 compound and one usually has no difficulty in recognizing it 

 and applying at once the name that has been agreed upon for 

 ■ular compound. But a rock is usual! v a mechanical 

 mixture of two or more minerals, able varia- 



tion in the relative amounts of the different constituents, re- 

 geologist substantially the 



rock. By the "adaptability 



a, A change in the re . 



should adapt ita n. A change in the relati 



of the 



limits should i 



It is here that I conceive some may take issue with me as to 

 the practicability of upholding this principle. It will be con- 

 tended that rocks grade >il m . . ,, ( othei In insensible transi- 

 tions and that the transitional forms arc just as common as those 

 we may choose to set up as typical forms ; that a name conse- 

 quently cannot be made to adapt itself to this indefinitely shift- 

 ing mixture of minerals. The answer to this has been so finely 

 stated by Eosenbusch (Massige Gesteine, p. 25) that I cannot 

 refrain from quoting it in full, premising that 1 have fully ex- 

 perienced the truth of his statement. 



He says : "In the same \vn } -'it ions from 



one rock to another are brought about by change in the min- 

 eralogical composition, so also is it true that similar transitions 

 are sometimes the results of a change in texture. I believe 

 myself justified in the statement that every thoughtful petro- 

 grapher will have passed through three stages of development 

 in his scientific views, with increasing experience in this direc- 

 tion, stages which possess more than a personal interest, from 

 the fact that they are likewise recognizable as three stages in 

 the development of the science itself. In the first of these 

 stages, one is a fanatic for a simple system ; one recognizes a 

 series of well characterized types and no transitional forms. 

 In the second stage, the conviction presses itself gradually upon 

 the mind that nature cannot be fitted into so exact and rigid a 

 frame; the well characterized rock-types become fewer and 

 fewer, the transitional forms more and more numerous. One 

 turns away from the rigid system and becomes an inspired ad- 

 vocate of the uninterrupted series, of the gradual development 

 of one rock from another, of I tions which 



bind together and totally obliterate the types of the first stage. 

 Finally, in the last stage, one begins gradually to discover cer- 



