XCvi PEOCEEDIXas OF THE aEOLOPTCAL SOCIETY. ' vol. IxXUl. 



the only selective action determining the geographical distribution | 

 o£ different classes of igneous rocks is not claimed. In pai-ticular, " 

 any discussion of that question as a whole must try to assess the 

 importance of isostasy, and the consequent tendency of the denser 

 basic magmas to accumulate beneath the ocean-basins. This and 

 some other elements of tlie larger problem may be eliminated in 

 considering so small an area as the British Isles ; which neverthe- 

 less can .still produce, as I have shown, not a few unsolved questions 

 to engage the attention of petrologists in the future. 



