Prof. S. J. Shand — Saturation in Petrography. 487 



" The ends of scientific classification are best answered when the objects are 

 formed into groups respecting which a greater number of general propositions 

 can be made, and those more important, than could be made respecting any 

 other groups into which the same things could be distributed. The properties, 

 therefore, according to which objects are classified should, if possible, be those 

 which are causes of many other properties ; or, at any rate, which are sure 

 marks of them. Causes are preferable, both as being the surest and most 

 direct of marks, and as being themselves the properties on which it is of most 

 use that our attention should be strongly fixed. But the property which is the 

 cause of the chief peculiarities of a class is unfortunately seldom fitted to serve 

 also as the diagnostic of the class. Instead of the cause, we must generally 

 select some of its more prominent effects, which may serve as sure marks of 

 the other effects and of the cause. 



" A classification thus formed is properly scientific or philosophical, and is 

 commonly called a Natural, in contradistinction to a Technical or Artificial, 

 classification or arrangement. The phrase Natural Classification seems most 

 peculiarly apijropriate to such arrangements as correspond, in the groups which 

 they form, to the spontaneous tendencies of the mind, by placing together the 

 objects most similar in their general aspect ; in opposition to those technical 

 systems which, arranging things according to their agreement in some 

 circumstance arbitrarily selected, often throw into the same group objects 

 which in the general aggregate of their properties present no resemblance, 

 and into different and remote groups others which have the closest similarity. 

 It is one of the most valid recommendations of any classification to the 

 character of a scientific one, that it shall be a natural classification in 

 this sense also ; for the test of its scientific character is the number and 

 importance of the properties which can be asserted in common of all objects 

 included in a group ; and properties on which the general aspect of the 

 things depends are, if only on that ground, important as well as, in most 

 cases, numerous." 



The properties upon which the general aspect of a rock (as 

 distinguislied from a rock-body) depends are its mineral constitution 

 and its structure (texture) ; hence it is not surprising that these 

 properties have from the first been seized upon as being most suitable 

 for the formation of groups. Whatever views one may hold as to the 

 general utility of a classification which is based upon these properties, 

 there can be no question about its being a perfectly ' natural ' one, and 

 therefore, according to Mill, a scientific one. 



But a school has arisen in later years which demands more regard 

 for chemical composition in our system, and which will not be 

 satisfied with the stock reply that "mineral constitution is a function 

 of chemical composition". Classifications have accordingly been 

 evolved in wliich mineral constitution is subordinated to iiltimate 

 composition, expressed either in molecular ratios or sometimes in 

 mere bald percentages. After much controversy it has generally been 

 conceded that there is reason in both views, and consequently in many 

 recent attempts at classification it has been sought to combine chemical 

 with mineralogical data. 



I am in full agreement with Mr. Scott when he states that " These 

 two factors . . . the chemical and mineralogical composition, are the 

 only ones we have for a basis in classification ", but when lie goes on 

 to assert that they "must be correlated", then I stop short to ask 

 liim how he proposes to accomplish it. It is so easy to say " let it 

 be done", so difficult to do it ! And Mr. Scott gives us no assistance 

 whatever. 



