INTRODUCTION. 9 



garded as the eutectic plus (m i) b plus (n i) c. In some cases 

 at any rate the ground mass of a rock (as Mr. Teall pointed out) repre- 

 sents an eutectic. This is probably not true in general but, if it 

 were, the scheme proposed would be to group together in one genus 

 all the rocks which have the same ground mass and to regard the 

 phenocrysts as minor or specific characteristics. The size of crystals 

 is no index of the rapidity of their formation, and Messrs. Day and 

 Allen have shown that anorthites of the size of very large pheno- 

 crysts may form in a few minutes, while in more viscous magmas 

 small feldspar crystals may form with extreme slowness. Hence 

 great care is requisite in deciding microscopically the question which 

 crystals were the last to form. 



It is worthy of note that the geological behavior of an intrusive 

 or effusive rock is conditioned largely by the character of the eutectic. 

 So long as this remains liquid the phenocrysts are, mechanically speak- 

 ing, mere flotsam and jetsam in the stream. The character of the 

 eutectic must decide whether a lava pours down a gentle declivity as 

 does a basalt, or piles up about the orifice like a rhyolite. Now, if it be 

 not essential to consider such geologically important properties in the 

 classification of rocks, it is at all events desirable to do so. Such 

 properties must sooner or later be dealt with methodically by geolo- 

 gists, and a thoroughly rational classification of rocks will correlate 

 physical and chemical properties. 



These last paragraphs deal with plans rather than achievements, 

 and have been written chiefly to emphasize the importance of the 

 work done by Messrs. Day and Allen as one step in a broader scheme. 

 Evidently every step of the larger plan involves accurate studies of 

 the melting points and thermal properties of the rock-forming min- 

 erals, and first of all that most important group, the lime-soda feld- 

 spars, which make up approximately one-half of the lithosphere. In 

 the meantime, since my proposal to use eutectics as a basis of rock 

 classification was printed, some valuable work has been done on 

 eutectics, chiefly by J. H. L. Vogt.* 



Only the first step has been taken in this investigation the study of 

 the triclinic feldspars in dry fusion. It has been attended with great 

 difficulties, many of them only touched upon in the paper which fol- 

 lows, but of which I have been cognizant in detail. Except for Dr. 

 Day's resourcefulness and experimental skill, success would not have 

 been achieved, but a road has now been broken out in this ultra- 



* Die Silikatschmelzlosungen. Christiania, 1903. Mr. Vogt printed a pre- 

 liminary communication early in 1902. 



