Vol. 50.] BASIC ERUPTIVE ROCKS OE GRAN. 37 



So far, I think, we are on safe ground ; I would expressly point 

 out that I have not discussed the primary reason for a differentiation 

 of the above-described nature, whether this is to be sought for in 

 Soret's principle, in the effect of chemical affinity, or in other causes. 

 That in many cases also the principle of Guy and Chaperon, the 

 commencement of crystallization and the sinking to the bottom 

 of the crystallized masses, as well as a subsequent re-melting 

 of such early crystallizations, may have performed a part in the 

 processes of magma-differentiation is quite possible. We move here 

 in a maze of hypotheses. 



But the differentiation itself is not a hypothesis ; it must now be 

 reckoned with as a solid fact of great importance. The same law, 

 which in a narrow dyke-fissure has produced a differentiation 

 along the more rapidly cooling dyke-sides, has, operating on a 

 larger scale in the magma-basins in the earth's crust from which 

 eruptions of a local volcanic centre originate, differentiated out 

 the pressed-up secondary magmas, so that they have succeeded 

 each other in a regular order. Finally, the same law has perhaps 

 determined the particular composition of the magma of each separate 

 magma-basin by differentiating the same out of the pristine liquid 

 magma upon which, by the cooling of the earth, the solid crust was 

 deposited. 



Discussion-. 



The President said it was of advantage to the Society to have 

 communications of this kind from distinguished foreign geologists. 

 The paper reminded him of one by Dakyns and Teall, published 

 in Quart. Journ. vol. xlviii. (1892). The basis of this philosophy 

 appeared to lie in the determination of the order of crystallization by 

 the microscope. The theory must be supported by very clear field- 

 evidence, otherwise it would remain a mere speculation. 



Prof. Judd said that the Geological Society of London must hail 

 with pleasure the fact that Prof. Brbgger had chosen their Journal 

 as the means of communication to the world of a memoir of such 

 value and interest. Prof. Brogger's contributions to all branches 

 of geological and mineralogical science are so large in amount and 

 invaluable in character that his claims on the attention of geologists 

 are unrivalled. The speaker especially referred to the novelty and 

 interest of the Author's views concerning the mode of separation 

 of magmas, and to his suggestion that the nature of contact- 

 metamorphism depends on the character of the erupted rock, as 

 well as on the materials through which it has been ejected. 



Gen. M°Mahon remarked that the Author appeared to hold the 

 view that the differentiation of a magma into a continuous series of 

 rocks, ranging from those of a more basic to those of a more acid 

 type, depended on the laws that determine the sequence of crystal- 

 building — that is to say, that the more ba^ic minerals are those 

 which first crystallize out from a magma, the remaining minerals 



