GENERAL CONCLUSIONS llo 



3. Many of the so-called siibalkaline rocks are richer in soda and potash 

 than many of the so-called alkaline rocks. There is no a priori reason 

 why, under certain conditions, subalkaline magmas should not yield typi- 

 cal alkaline rocks by diiferentiation. For example, a small body of pho- 

 nolite could be differentiated from a large body of basaltic magma with- 

 out giving a femic pole strange to petrography. 



4. Many alkaline igneous types are chemically and mineralogically 

 transitional into subalkaline types. 



5. At least two-thirds of the alkaline rock districts of the world contain 

 thick masses of limestones, dolomites, or other calcareous sediments which 

 have been cut by the alkaline eruptives. For the remaining cases, except, 

 possibly, the Cripple Creek district, information is lacking as to the pre- 

 cise nature of the intruded formations. 



6. Many of the minerals special to the alkaline rocks are among those 

 known to form in silicate melts or magmas which have dissolved lime- 

 stone or other carbonate. The list of such minerals includes primary can- 

 crinite, primary calcite, haiiynite, melilite, scapolite, wollastonite, gar- 

 nets, titanite, graphite, analcite, perovskite, etcetera. 



7. The most typical alkaline rocks are so low in silica that feldspa- 

 thoids, instead of feldspars, crystallize out. 



The theoretical suggestions derived from these facts may be likewise 

 summarized : 



1. The alkaline rocks have been derived from the steadily associated 

 subalkaline magmas. Phonolites, many nephelite syenites, nephelite ba- 

 salts, nephelinites, leucite basalts, leucitites, melilite basalts, tephrites, 

 basanites, trachydolerites, theralites, etcetera, appear to be genetically 

 connected with common feldspar basalt. In some cases, like the nephelite 

 syenites of eastern Ontario, typical alkaline bodies are syngenetic with 

 granites, which themselves, in probably all instances, cut calcareous sedi- 

 ments. 



2. The segregation of the alkalies and the observed desilication of the 

 subalkaline magmas are largely explained by the assimilation of the natu- 

 ral carbonates. It is conceived that the whole of the carbonate may take 

 part in the appropriate chemical reactions; or that sometimes the sub- 

 alkaline magma is affected only by the carbon dioxide which is driven off 

 from invaded limestone by magmatic heat into the volcanic vent or other 

 magmatic chamber. Various suggestions are offered as to the nature of 

 the reactions, but they are all strictly tentative and need checking by 

 physico-chemical experiments. Evidence is given for the view that, inde- 

 pendently of the other reactions, one or both of the principal alkalies are 

 caused to migrate positively, giving such types as tephrites and leucite 



