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



a given rock. The terms saturated, unsaturated, and tlieir derivatives 

 overs aturaied, undersaturated, and partsaturated, were not used in 

 a strict chemical sense, but in a special mineralogical sense which 

 was fullj'^ explained. 



Mr. Alexander Scott' has criticized my proposal from a theoretical 

 standpoint, and rejects it, but omits to say whether he prefers the 

 ' silica-percentage ' type of petrography which it was my aim to 

 supplant. It was not my intention, in writing the paper in question, 

 to offer a complete solution of the most vexed question in petrography, 

 but only to contrast the efficacy of the 'saturation' criterion with 

 that of the silica-percentage as the basis of a descriptive classification. 

 I showed, to my own satisfaction at least, that the former has various 

 advantages over the latter, its easy applicability to the majority of 

 rocks being only one of them. He, however, raises the whole 

 question of the classification of igneous rocks, and subjects my 

 proposal to criticisms which are equally applicable to all propositions 

 for rock classification which have ever been made. 



Mr. Scott dreams of a classification which shall "correlate rocks 

 not only of similar chemical composition but also with similar cooling- 

 'histories' ". So do vpe all, and we shall all wish with him success in 

 the search for it. But in the meantime such a classification eludes 

 us, and we must live in a cottage until the palace is ready. It is 

 poor work to destroy without creating, so until he is able to give us 

 something better in exchange for it, we shall beg Mr. Scott (who is 

 only following in the footsteps of Cross and many another) not to pull 

 our present refuge down about our ears. As for my own little 

 contribution, which started this discussion, I regard it in the light of 

 a new beam inserted in the temporary structure in the place of one 

 which, in my judgment, was defective ; I have not sought to lay the 

 foundation of a new edifice. When the ultimate classification of 

 rocks is forthcoming, my suggested groupings will disappear from 

 currency (should they ever attain it) ; nevertheless, I have sufficient 

 confidence in their value to believe that they will not be found 

 irreconcilable with, the fundamental principles of that classification of 

 the future. 



When one approaches the subject of rock- classification, it is 

 very important to have a clear idea of the aim of classification 

 in general. John Stuart Mill's views on the subject cannot be 

 surpassed for clearness, and the following extracts from his System 

 of Logic seem to me to be so especiallj' pertinent to the present 

 subject that I offer no apology for quoting them in a petrographical 

 discussion. 



" Classification ... is a contrivance for the best possible ordering of the 

 ideas of objects in our minds ; for causing the ideas to accompany or succeed 

 one another in such a way as shall give us the greatest command over our 

 knowledge already acquired, and lead most directly to the acquisition of more. 

 The general problem of Classification, in reference to these purposes, may be 

 stated as follows : To provide that things shall be thought of in such groups, 

 and those groups in such an order, as will best conduce to the remembrance 

 and to the ascertainment of their laws. 



1 Geol. Mag., July, 1914, p. 319. 



