Reviews — A New Rock- Classification. 175 



terminology introduced, is not a just one. The principles upon 

 which the whole assemblage of igneous rocks is divided and sub- 

 divided are, at least on the face of things, perfectly simple, and they 

 ^re carried out logically and consistently throughout. The criteria 

 appealed to are, moreover, of a strictly quantitative kind, so'that all 

 ambiguity and compromise are excluded, manifestly an advantage if 

 our object is to provide each rock with a precise systematic position 

 and name. It follows, of course, since the hard dividing lines have 

 no counterpart in nature, that rocks having the closest affinities 

 mast sometimes be divorced ; but this is a drawback incident to any 

 arbitrary scheme. 



The basis of classification is mineralogical composition, though 

 this rough statement needs, as we shall see, important qualifi.cation. 

 The mineralogical criteria selected are such as correspond with more 

 or less important differences among the rocks themselves, and those 

 which are deemed of most importance are used in making the major 

 divisions of the scheme, the principle being carried out, as it seems 

 to us, with skill and judgment. Thus the rock-forming minerals 

 are first grouped under two heads, the siliceous and aluminous on 

 the one hand and the ferro-magnesian on the other, or, as we 

 may conveniently say, the light and the dark. The whole of the 

 igneous rocks are then divided into five great classes, according 

 to the ratio which the total light minerals bear to the total dark ; — 



(i) Light minerals extremely abundant ; ratio greater than 7 : 1. 

 (ii) Light minerals dominant ; ratio between 7 : 1 and 5 : 3. 

 (iii) Light and dark minerals nearly equal ; ratio between 5 : 3 



and 3 : 5. 

 (iv) Dark minerals dominant ; ratio between 3 : 5 and 1 : 7. 

 (v) Dark minerals extremely abundant ; ratio less than 1 : 7, 



This we may regard as in some sense an extension of Brogger's 

 leucocratic and melanocratic groups. In the first three classes 

 subdivisions are made with reference to the nature of the white 

 minerals, five subclasses being defined in each class according to the 

 ratio of quartz, felspars, and felspathoids on the one hand to 

 corundum and zircon on the other. In the last two classes, 

 where dark minerals preponderate, these are made the basis of a like 

 subdivision, the critical ratio adopted being that of pyroxenes, 

 olivine, and magnetite to apatite, etc. This produces an ill-balanced 

 arrangement, for it is evident that in every class the first subclass 

 must greatly outweigh all the others. It is, however, easier to 

 criticise the scheme in these particulars than to improve it. It is 

 not possible here to follow out all the details of this ingenious 

 arrangement, and we will only note how the subclasses are divided 

 into orders. In the first three classes this is effected with reference 

 to the preponderance of quartz, felspars, or felspathoids among the 

 white minerals. Since quartz and the felspathoids are ' antithetical,' 

 not being found in company in igneous rocks, it is possible here to 

 use at once the ratios of quartz to felspars and felspars to felspathoids ; 

 which leads to nine orders in each subclass, the dividing ratios being; 



