Professor S. J. Shand—A System of Petrography. 467 



4. We have next to attend to the colour-ratio — that is, the ratio 

 of the heavy, mostly dark-coloured minerals to the light, mostly 

 pale-coloured minerals. The really important point ahout them is 

 of course their density, but the colour is a convenient mark of it. 

 I have already proposed the use of a set of prefixes to indicate this 

 character. 1 I would go further than this, and actually give different 

 names to rocks with conspicuously diffei-ent values of the colour- 

 ratio. This is already done sometimes, as in the following cases: — 



orthoclasite — > syenite — > shonkinite — > perknite 

 anorthosite — > gabbro — >■ (picrite) — > pyroxenite, etc. 

 alaskite — ^- granite — > 

 To make such a distinction quantitative, it would accord fairly with 

 existing practice if we fixed the boundaries at 3, 50, and 97 per 

 cent of heavy minerals. In the oversaturated division it is obvious 

 that the melanic end-member of each family would be a quartz- 

 perknite. In the S and U divisions it would be a pyroxenite or 

 hornblendite. In the u and W divisions the corresponding end- 

 member would be a peridotite or cromaltite. 2 All the varietal names 

 required for the more minute description of these are already in 

 existence. For the leucocratic end-members too it will rarely be 

 necessary to find new names, since nearly all cases are already 

 covered by such names as orthoclasite, albitite, oligoclasite, anor- 

 thosite. It is mainly for the third group, containing from 50 to 97 

 per cent of heavy minerals, that new names analogous to shonkinite 

 will be required. Owing to the similarity of the end products, the 

 total number of distinct groups will not be 8x4, but practically 

 about 20 in each division. The four groups in each family may be 

 distinguished by the letters L, 1, m, and M where suitable names are 

 not at present in existence. 



5. As regards crystallinity, I have a decided preference for the 

 two groups of Zirkel (and Iddings) as opposed to the three groups of 

 Rosenbusch (and Holmes). As everybody knows, there is no special 

 set of characters which one can postulate of a so-called dyke rock — 

 not even that it occurs in a dyke. Some dyke rocks have all the 

 characters of plutonics, others all those of lavas. There are in fact 

 only two sets of cooling conditions which produce really significant 

 differences in the characters of the rocks formed under them; these 

 conditions are slow cooling with a sufficiency of fluxes and rapid 

 cooling with diminished content of fluxes : that is, plutonic and 

 effusive conditions, giving rise to phanerocrystalline and aphanitic 

 rocks respectively. If it is desired to express the actual manner of 

 occurrence of a rock mass, it can be done by means of a prefix 

 attached to the name, as dyke-granite, dyke-rhyolite, and so on. 

 A tinguaite I would describe as a dyke-phonolite, a grorudite as 

 a dyke-ekerite or microekerite, a vogesite as a dyke-shonkinite, 

 a hedrumite as a micronordmarkite, etc. In this way a great 

 number of superfluous names might be eliminated and others reduced 

 in value from group names to mere varietal names. 



1 Journ. Geol., 1916, p. 400. 



2 Melanite-pyroxenite : Trans. 



Geol. Soc. Edin., 1910, p. 376 



