TABLE or FIELD ASSOCIATIONS 91 



cent of potash. If alkalies sufficient for one volume of plionolite were 

 concentrated from twenty volumes of basalt, the residual magma would 

 still have a composition little different from that of average basalt. The 

 total volume of the knoAvn phonolites is surely not more than five per 

 cent of the known volume of extruded basalt. Much slighter concentra- 

 tion of the alkalies from basalt would give the soda and potash appro- 

 priate to trachydolerite, tephrite, or basanite. These illustrations show 

 that there is no violence done to the principle of magmatic differentia- 

 tion in making it responsible for the derivation of alkaline rocks from 

 subalkaline magmas. That derivation is further suggested by the nu- 

 merous transitions observed between alkaline and subalkaline rocks. 



Why is the eruptive sequence of subalkaline magmas ever broken? 

 What is the cause of the concentration of their alkalies so that at certain 

 times and places relatively small eruptions of alkaline magmas occur? 

 The attempt to answer these questions is the central theme of this paper. 



Association of alkaline Rocks witli Limestones, Dolomites, and 

 other calcareous sediments 



The following table gives a summary statement of the facts from which 

 a general rule connecting alkaline igneous rocks and calcareous sediments 

 may be deduced. With but few exceptions, if any, the magmas from 

 which these eruptive rocks were derived cut limestone, dolomite^ or other 

 deposits containing notable proportions of carbonate.^ 



The sources of information used in making the table were reached 

 chiefly through the lists of references published in the fourth edition of 

 Eosenbusch's "Mikroskopische Physiographic der Massigen Gesteine." 

 It was found that most of the petrographic papers give few facts regard- 

 ing the formations cut by the alkaline eruptives. In many instances the 

 more purely geological reports, memoirs, and maps have given valuable 

 data on the subject. These additional references are usually the obvious 

 ones ; to keep this paper within a proper limit of size, they are not listed. 

 The table is not complete, though it contains names of nearly all the. 

 important alkaline-rock districts now known on the globe. 



In constructing the fourth column, it was often necessary to search for 

 information as to the sedimentary terranes underlying the present land 

 surface in the districts. Care was taken to assume limestone below the 

 surface only when the facts pointed strongly in that direction. It was 



» A remarkable Illustration Is found In the great nephellte syenite fields of eastern 

 Ontario, where the foyaltlc phase of the pre-Cambrlan granitic hathollths is found at 

 contacts with limestone. F. D. Adams and A. E. Barlow, Transactions of the Royal 

 Society of Canada, third series, vol. 2, section 4, 1909, p. 8. 



