In exercising a debater's right to reply to criticism, I shall be extremely brief. My contentions, in two essays on the above subject,1 were as follows:—

1. That the classification of rocks by silica percentage is unsatis-factory, and that a more natural basis for classification is available in the degree of saturation of the constituent minerals.

My critic, Mr. Scott, has not expressed any opinion on this pointof comparison.

2. That the field evidence is overwhelmingly favourable to the separation of the rock minerals into two groups, those of one group being stable in the presence of free silica under magmatic conditions, those of the other group unstable; and that such experimental work as has been done supports that separation.