BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 21, pp. 87-118 March 31, 1910 



OEIGIN OF THE ALKALHSTE EOCKS^ 



BY REGINALD A. DALY 



{Presented in abstract before the Society December 30, 1909) 



CONTENTS 



Page 



Definition 87 



General statement of the investigation 8H 



Association of alli:aline and suballvaline roclis 81) 



Allvaline rocks genetically connected with subalkaline magmas 00 



Association of alkaline rocks with limestones, dolomites, and other c;il- 



careous sediments i) 1 



Table showing tield associations of alkaline and subalkaline eruptives and 



calcareous sediments 92 



Effects of the vsolution of carbonates in subalkaline magma 108 



Additional evidences favoring the hypothesis .\ . 110 



Possible objections to the hypothesis Ill 



Alkaline rocks not associated with calcareous sediments 113 



General conclusions 114 



Definition 



Alkaline rocks include two classes : first, those relatively rich in soda, 

 in potash, or in both ; and, secondly, those which, with relatively low total 

 of alkalies, yet carry essential amounts of minerals specially character- 

 istic of alkali-rich eruptives, such as nephelite, leucite, analcite, etcetera. 

 Field (genetic) association is also a criterion generally used in separating 

 any alkaline type from the subalkaline or lime-alkali group of igneous 

 rocks. Thus, monzonite may not be rich in alkalies nor carry any feld- 

 spathoid, 3^et it is regarded as of the alkaline group largely because it is 

 often in comagmatic relation to nephelite syenite or the alkaline feldspar 

 syenites. For the same reason, camptonite, alnoite, limburgite, and 

 melilite basalt with, on the average, only 4.1 to 5.4 per cent of alkalies for 

 each, are considered as belonging in the alkaline groups, though no feld- 

 spathoid is an essential constituent of any one of these rocks. On the 



Manuscript received by the Secretary of the Society January 4, 1910. 



(87) 



