128 A. Holmes — Classification of Igneous Rocks. 



Or 87 . 5 . Some authors have used 1/3-2/3 (Or 33 -Or 67 ) as the limiting 

 ratios of orthoclase to total felspar in certain groups of rocks such 

 as monzonite (Hatch, 1916, vol. i, p. 192). This procedure, 

 however, compares orthoclase to soda-lime felspar, and as the 

 composition of the latter varies according to its occurrence in (say) 

 granites or gabbros, adamellites or labradorite-monzonites, it follows 

 that the same factors are not used throughout. Iddings in his modal 

 classification (vol. ii) adopts 3/8-5/8 as limiting ratios, with a still 

 greater possibility of variation in the factors, since for orthoclase 

 he substitutes alkali felspar. 



Until we know whether there be a natural grouping of rock types 

 about certain points (and relative abundance of the types concerned 

 must, of course, be the chief test applied), the precise value of the 

 limiting ratios adopted does not seriously matter. Those here 

 employed are the ratios of the C.I.P.W. Classification, using in 

 general only three of the five divisions. The separation of grano- 

 diorite from quartz-diorite and adamellite demands a fourth sub- 

 division. According to the definition of Lindgren, the orthoclase 

 limits for granodiorite are about Or, — Or 33 , consequently the C.I.P.W. 

 limits Or 12 . 3 — Or 37 . 5 will serve equally well. In the other direction, 

 a fifth subdivision is necessary to accommodate the labradorite- 

 monzonites which, starting at Or 37 . 3 , may be allowed to pass over 

 the Or 62 . 5 dividing-line as far as the Or s7 . 5 limit. These limits are 

 broader than are actually required, but as they do no violence to the 

 definition of a gabbro, there can be no objection to their adoption. 

 The writer does not wish to insist on rigidly fixed lines of division, 

 for it is his opinion that a really valuable quantitative classification 

 can only emerge when thousands of modes have been measured and 

 statistically examined. The mode is the only "symbol " that can at 

 present usefully be given to a rock. Nevertheless, for a tabular 

 statement to be possible at all, lines must be drawn somewhere, and 

 those adopted seem to enclose all accepted rock names without 

 changing their current significance. 



There remain to be considered rocks without actual felspar. 

 Logically, the five divisions here made on successive pages should be 

 repeated for rocks free from felspar, the divisions giving in respective 

 order : Quartz rocks, Pyroxenites and Hornblendites, Peridotites, 

 Pelspathoid rocks, and Olivine-felspathoid rocks. It is much more 

 convenient, however, to treat each group as a limiting case of the 

 felspar group to which it most clearly belongs. This is done in 

 the tabulation by placing some of the rocks in question below 

 the corresponding felspathic rocks. 



In each division of the tabular scheme, the coarse-grained rocks 

 (generally those of major intrusions, to use Dr. Evans' convenient 

 term for ' plutonic ' masses) are used as types, and allied porphyritic, 

 fine-grained, or aphanitic varieties (belonging generally to minor 

 intrusions and lava-flows) are grouped with them. 



The classification is printed in three dimensions, in sheets super- 

 imposed one on another as required. Of these dimensions, the first 

 is based on degree of silica-saturation, and since it involves lines of 

 variation from saturated types in the directions respectively of quartz, 



