Professor S. J. Shand — A System of Petrography, 463 

 IV. — A System of Pethogbafhy. 

 By S. J. Shand, D.Sc, F.G.S., University of Stellenbosch, South Africa. 



fPHE recent paper by A. Holmes on "A Mineralogical Classification 

 J_ of Igneous Rocks " l is an attempt to settle a matter which 

 English-speaking petrologists ought to have tackled long ago. There 

 seems, however, to be a disposition, on the part of British geologists 

 especially, to leave the matter alone in the fond belief that some 

 "natural" system of classification will presently be revealed to us 

 whereby all our taxonomic doubts and difficulties will be dispelled. 

 Nothing can be more unreasonable. The philosophic treatment of 

 a subject must follow, not precede, the preparatory work of 

 discriminating, sorting, and docketing; and the sharper the dis- 

 crimination, the more thorough and orderly the sorting and docketing, 

 the easier to make the philosophic digest. Hitherto petrographer& 

 have not discriminated well; we have not done our sorting in 

 a methodical way ; we have tied on labels in the most haphazard 

 fashion; our house is in a state of dreadful confusion. It is true 

 that through all the disorder we have caught fascinating glimpses of 

 regularities and relationships ; of connexion between this and that; 

 but the deplorable fact appears that we are unable to test or verify 

 any hypothesis, or to take a single confident step towards the 

 philosophic treatment of our subject, until we put our house in 

 order. We have got to do our sorting and docketing all over again, 

 and do it methodically, and then only can we begin to look for the 

 philosophy of the matter. 



I have already taken in this journal what I consider to be the first 

 step in the process of bringing order out of confusion — I refer to the 

 separation of five prime divisions of igneous rocks on the basis of 

 the degree of saturation of their constituent minerals. 2 I should 

 in the near future have tried to indicate some further steps in the 

 same direction, but I wished to present a complete scheme, with 

 illustrative examples, and this, with the imperfect facilities of 

 a Colonial university, has taken much time. Now that Mr. Holmes 

 has broached the question, however, I think it ought not to be 

 allowed to drop until some understanding is reached, and I wish 

 therefore to submit an alternative system in very brief outline. 



Holmes adopts my five saturation classes as the basis of his 

 arrangement, and then effects a first subdivision within each class 

 by means of a cross-classification in which the orthoclase-albite ratio 

 is put against the albite-anorthite ratio. In arriving at the former 

 of these ratios he proposes to calculate the felspathoids, micas, and 

 analcite into their equivalent amounts of orthoclase and albite. 

 Following upon this he makes the histoi'ic separation of plutonic rocks 

 from dyke-rocks and lavas; and to establish still smaller subdivisions 

 he suggests (but does not elaborate in his paper) the employment of 

 the colour ratio (felsic-mafic ratio) and the texture, in that order. 



1 Geol. Mag., March, 1917, p. 115. 



2 Geol. Mag., November, 1913, p. 508 ; November, 1914, p. 485 ; August, 

 1915, p. 339. 



