摘要
Since the publication of Volumes Ⅰ and Ⅱ of this Petrography, I have given very serious consideration to the question of changing the limits of the various classes in the rock classification from 0-5-50-95-100 per cent dark minerals to 0-5-45-85-100 per cent, 0-10-40-70-100 per cent, or some such values, to make Class 3 embrace rocks a little less melanocratic than fixed by the 50-50 point, and Class 4 embrace rocks with a little higher percentage of leucocrates.