550 



SCIENCE. 



[N. S. Vol. XX. No. 513. 



explanation of fusion is concerned. Only 

 those who have studied the minute evi- 

 dence of mechanical action in mountain 

 ranges can appreciate the evidence they 

 present of stupendous dissipation of en- 

 ergy. This has not indeed been enough 

 to fuse the rocks, but it is hard to conceive 

 that it is always insufficient to furnish the 

 latent heat of fusion to rocks already close 

 to their melting point under the prevailing 

 pressure. From this point of view, vul- 

 eanism is a feature of erogenic movement 

 and it is to be looked for where relative 

 motions are concentrated in zones so nar- 

 row that the local dissipation of energy is 

 relatively intense. It is also possible that 

 percolating waters, by reducing the melt- 

 ing points of rocks, sometimes bring about 

 fusion without change of temperature. 

 Such an hypothesis might fit the volcanoes 

 of the Hawaiian islands where there is no 

 known faulting in progress. 



The physics of magmatie solutions is a 

 great subject which is experimentally 

 almost untouched, although a vast amount 

 of geological speculation has been based 

 upon assumed properties of magmas. It 

 is only within a few months that even satis- 

 factory melting-point determinations of 

 those most important rock-forming min- 

 erals, the lime-soda feldspars, have been 

 made. The feldspars are only one series 

 of isomorphous mineral mixtures. Their 

 study is fundamental and must be fol- 

 lowed by that of the remaining class, i. e., 

 the eutecties. These, in my opinion, will 

 lead to a rational classification of igneous 

 rocks, themselves mixtures and incapable 

 of logical description except in terms of 

 standard mixtures, the eutecties. 



It appears to me highly probable, for 

 many reasons, that the magmas of the 

 granular rocks are not liquids but stiff 

 emulsions, comparable Avith modeling clay, 

 the solid constituents (perhaps free oxides) 

 being merely moistened with magmatie 



liquids. Such masses behave mechanically 

 like soft solids ; they display some rigidity 

 and in them diffusion is reduced to a van- 

 ishing quantity. They may be ruptured 

 and the (aplitic or pegmatitic) liquid por- 

 tion may then seep into the cracks. Such 

 a magma might be forced into minute fis- 

 sures, as is the case when clay is molded 

 to terra cotta articles and yet it would 

 support permanently, on its upper surface, 

 rocks of superior density. Only in such 

 a magma can I comprehend the simultane- 

 ous growth of crystals of various minerals ; 

 for in a liquid not exactly eutectie, the 

 formation of crystals must follow a definite 

 order. Again, if banded gneisses and 

 gabbros had been fluid, the bands would 

 show evidences of diffusion which as a rule 

 are absent or barely traceable in these 

 rocks. 



The relations between consanguineous 

 massive rocks have occupied a large part 

 of the attention of geologists for many 

 years. At one time it was supposed that 

 homogeneous liquid magmas might split up 

 into two or more homogeneous magmas by 

 processes of molecular flow due to differ- 

 ences of osmotic pressure. This process 

 was called the differentiation of magmas. 

 It has been shown, however, that these 

 processes are so much slower even than 

 heat diffusion, that they can not be efficient 

 beyond distances of a few centimeters. 

 For this reason, Mr. Teall,* who first sug- 

 gested the application of the Soret process 

 to account for differentiation. Professor 

 Broggerf and others, have abandoned the 

 hypothesis of differentiation on a consider- 

 able scale by molecular flow. Nevertheless, 

 observations on laccolites and other occur- 

 rences leave no doubt that a single magma 

 may solidify to different though consan- 



* Proc. Geol. Soc. London, Vol. 57, 1901, p. 

 Ixxxv. 



t ' Eruptivgesteine der Kristianiagebietes,' part 

 III., p. 339. 



