90 R. A. DALY ORIGIN OF THE ALKALINE ROCKS 



nephelite syenite "laccolith" of the Kola Peninsula, with an estimated 

 area of 1,600 square kilometers. The Kangerdluarsnk (Greenland) 

 nephelite syenite covers 390 square kilometers. The Bancroft (Ontario) 

 nephelite syenite covers about 25 square kilometers. These figures show 

 the order of magnitude of the largest intrusive alkaline bodies. The alka- 

 line extrusives are yet more numerous, but are generally of much smaller 

 volume than any of these plutonic masses. The visible alkaline rock of 

 an average locality covers but a few square kilometers, and its reasonably 

 inferred volume is only a few cubic kilometers. All the visible alkaline 

 rock of the world probably constitutes less than one per cent of the total 

 visible igneous rock. The visible volume of the subalkaline Coast Eange 

 batholith of British Columbia and Alaska, covering at least 150,000 

 square kilometers, is doubtless much greater than that of all the world's 

 known alkaline bodies put together. 



There seems to be no reason to doubt that the size of the erupted bodies 

 will be, on the average, in direct proportion to the size of the feeding 

 magma chambers. If each chamber be a primeval alkaline reservoir, it 

 has, accordingly, a very small size as compared with the known bodies of 

 subalkaline granite or basalt. We are led to the deduction that relatively 

 and absolutely small bodies of magma can conserve the heat appropriate 

 to fluidity for scores of millions of years. Simple calculation and com- 

 mon sense show that this is an impossible supposition. If we turn on our 

 tracks and deny the smallness of the alkaline-magma reservoirs we face a 

 dilemma as clearly unacceptable to the field geologist. 



Alkaline Eocks genetically connected with subalkaline Magmas 



If subalkaline magma alone carries enough primary heat to be erupti- 

 ble, it follows that alkaline magmas are either differentiates from subal- 

 kaline magma or syntectics,^ — that is, solutions of foreign rock in sub- 

 alkaline magma — or differentiates from such S3mtectics. For the first and 

 third of these h3'potheses the observed field association of the two rock 

 groups, the relative volume of each group, and the chemical composition 

 of the alkaline types are all more or less appropriate. We have seen that 

 many subalkaline rocks actually carry more alkalies than some "alkaline" 

 rocks. The extreme case of associated phonolite and basalt may be briefly 

 discussed in preference. Average phonolite (calculated as water-free) car- 

 ries about 9.0 per cent of soda and 5.3 per cent of potash. Average basalt 

 (calculated as water-free) carries about 3.2 per cent of soda and 1.6 per 



* This word Is adapted from Loewinson-Lesslng. Compte Rendu, 7e session, Congrfes 

 g^ologlque Internationale, Saint Petersburg, 1899, p. 375. 



