324 the rocks op la saline. [May 1 900, 



the history of those from La Saline can be obtained from evidence 

 which in itself is anything but clear. It has been pointed out in 

 the preceding pages that reconstitution of the minerals of included 

 fragments takes place by the introduction of constituents from the 

 involving magma ; that the dark patches found in granites may 

 result from the complete alteration of such inclusions ; and that, by 

 the dissemination of the minerals of the fragmentary patches thus 

 produced, new constituents may be added to the intruding rock. 



In conclusion I wish gratefully to acknowledge the help that Prof. 

 Bonney has afforded me in the preparation of these notes. 



Discussion (on the two poregoing Papers). 



Prof. Sollas said that he recognized the great value of the con- 

 clusions to which the Author had been led. The statement that 

 successive intrusions cannot be separated one from the other by 

 hard-and-fast lines suggested that the district was a somewhat 

 ''xceptional one. In most cases when a passage appeared to exist 

 between different kinds of rocks, even when these were as closely 

 allied as a granite-porphyry and a quartz-porphyry, patient exami- 

 nation would usually reveal a sharp line of demarcation between 

 them. As regards differentiation, the existence of ' mixture ' ex- 

 plained many cases, supposed to be due to this hypothetical process, 

 in another manner ; and the statement that the rocks described were 

 intruded in an order of increasing acidity seemed to gain little from 

 a translation into hypothetical terms. It must not be overlooked 

 that while in some districts the order of eruption was one of in- 

 creasing acidity, in others it was, on the contrary, one of increasing 

 basicity. 



As regards the solution of a dolerite by granite (suggested in the 

 paper on La Saline), it might be observed that the consolidation- 

 point of a granite was certainly lower than the fusion-point of a 

 basalt ; if, therefore, the latter has been dissolved by the former, the 

 solvent (granite) must have been in a state of superfusion, and its 

 differentiation could scarcely be explained as a result of progressing 

 crystallization. 



The President also spoke. 



Prof. Bonnet, in the absence of the Author, replied that the 

 latter's remarks on the difficulty of distinguishing cases of intrusion 

 applied to this particular locality, where it certainly existed. He 

 had always believed that basalt was a rock that was easily fused, 

 and was not prepared to accept Prof. Sollas's statement about it 

 and granite. The idea of differentiation played a very subordinate 

 part in the paper, and was only put forward as an hypothesis. 

 At any rate, it seemed not inconsistent with the facts. 



