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



members of the plagioclase group for which oligoclase and 

 ite stand as types, that most commonly occur. No 

 great element of uncertainty would be introduced into our 

 nomenclature if we considered plagioclase a single mineral 

 species, subject of course to variation in the amounts of soda, 

 lime and silica, and in the future progress of the science, as the 

 exact nature of the plagioi Iase for each individual case became 

 determined, the name of this variety could easily be substi- 

 tuted for plagioclase in the name of the rock, or used as an 

 adjective modifying the name, if a purely trivial one. We 

 should have, e. g. " oligoclase-basalt" and "labradorite-hasalt" 

 in the place of "basalt" as now used. It must certain I y b« 

 conceded (as indeed it is generally believed) that no weight 

 can attach to the objection that the constituent minerals of a 

 rock cannot be accurately determined. 



Supposing, now, that our nomenclature is to be based upon 

 purely mineralogical grounds, it becomes necessary to determine 

 what' minerals shall be utilized for this purpose. The mineral 

 constituents of a rock have been divided into primary and 

 S'.-rnnilrtry according as they are the product of immediate crys- 

 tallization out of the original rock-magma before or at the final 

 >n of the rock: or are produced l.y changes in the 

 rock subsequent to its final solidilication brought about by 

 atmospheric waters or local metamorphism. To these may be 

 added such minerals and fragments as may have been mechan- 

 ically enclosed during the process of eruption ; I shall call 

 such foreign minerals and fragments. With the fragments of 

 course we have nothing to do. 



Of these the secondary and foreign minerals should unques- 

 tionably be disregarded" in naming the rock, as in fact has 

 always been the practice. They are evidently purely adventi- 

 tious and have nothing whatsoever to do with the rock as such. 

 Their true nature can always be recognized and they conse- 

 quently introduce no element of uncertainty to create confu- 

 sion in names. There remain the primary constituents. Among 

 these a certain liability to confusion may exist in the distinc- 

 tion between essential and accessory constituents. The acces- 

 sory mineral of one rock is the essential mineral of another, 

 and just where to draw the line is not ulways at once evident 

 from the fact that an approximate <|uautitative estimate of the 

 minend has to be made. It is here that practice only can ren- 

 der skillful. From the nature of the case it can never be 

 otherwise, and serious embarrassment is not to be feared from 

 this source. It is to be presumed that geologists are to be 

 educated and the ability to make such distinctions mu9t be 

 learned. He who runs cannot expect to read. On the other 

 hand, it must be obs * from this 



