38 BASIC EEUPT1VE EOCKS OF GEAN — DISCUSSION'. [Feb. 1 894, 



following in the order of their basicity. Further, the Author held 

 that the more basic rocks were older — that is to say, were erupted 

 before the more acid ones ; the latter, as Mr. Teall explained, 

 forming dykes cutting the more basic rocks. If this were so, 

 it would follow that, as the differentiation in tbe general magma 

 progressed, the basic material would sink to the bottom and the acid 

 portion of the magma would remain at the top. When pressure was 

 exercised on a fluid, or viscid, magma differentiated into layers in 

 the way supposed, and eruptious began to take place, one would 

 have expected the acid top layer to have been the first to have 

 issued from the cauldron. It was a pity that the Author was not 

 present, as he (the speaker) would very much like to learn by what 

 physical process the deeper-seated and more basic portions of the 

 still fluid magma were first erupted. 



Prof. J. F. Blake and Mr. W. W. Watts also spoke. 



