ABSTRACT OF PROCEEDINGS. XXXIX. 



serve all three purposes at once. There follows an explan- 

 ation of how a genera] classification might render valuable 

 assistance in research, by compelling penologists to observe 

 properties in the field and laboratory, with regard to which 

 data are now very scarce. A general classification must 

 of necessity be transient, as the progress of science will 

 necessitate its being altered, at least in its lower ranks, 

 every few years.|The remainder of the paper deals specially 

 with the problem of the classification and nomenclature of 

 igneous rocks. The most important correlations for didactic 

 purposes are found to be : — mineral composition with 

 abundance, and mineral composition with chemical com- 

 position ; those for research : — chemical composition with 

 habit (i.e. shape, size, and depth below original surface) of 

 mass, composition of the rock with the composition of 

 neighbouring or connected igneous masses, and composition 

 with origin (genetic). Each correlation consists of a pair 

 of properties, and the property of each pair to be selected 

 as a basis of subdivision in the general classification is 

 decided on considerations which follow. For didactic pur- 

 poses the more easily observed of the pair should be chosen; 

 for research, the one which most requires to be observed. 

 On these grounds the author selects for didactic purposes, 

 mineral composition ; for research, community of origin 

 (so far as possible) and habit. It is noted that mineral 

 composition and chemical composition are really groups of 

 very numerous properties, for many minerals or oxides may 

 be taken as bases of subdivision, either as ratios, percen- 

 tages, or by the presence or absence method. 



The author concludes that the most convenient general 

 classification for the present time would be one constructed 

 as follows : — 



Rank I.: Based on alkali-lime-content of principal and 

 minor mineral constituents. Produces 2 Series : alkaline 

 and calcic. 



