116 R. A. DALY ^^ORIGIN OF THE ALKALINE ROCKS 



basalts from the difterentiated syntectic of carbonate with feldspar basalt. 

 It is conceivable that dolomite and pure limestone have different effects 

 in the reactions, the one concentrating more soda, the other more potash. 



3. The amount of natural carbonate required to develop an alkaline 

 differentiate is seen to be small, as compared with a given volume of sub- 

 alkaline magma. 



4; The minerals peculiar to, or characteristic of, alkaline rocks find 

 explanation on the hypothesis proposed. 



5. Many alkaline granites and feldspar syenites have no apparent gene- 

 tic connection with limestones, and to some of these igneous types the 

 carbonate-syntectic hypothesis is not extended. They are often, however, 

 differentiates from subalkaline magmas like granodiorite, etcetera. Many 

 of the syenites cut thick series of sediments other than limestone. The 

 solution of these other sediments with subalkaline magma must upset its 

 chemical equilibrium. Whether alkaline differentiates would result is a 

 question the discussion of which is fraught with yet greater physico- 

 chemical difficulties than that above outlined. No attempt is made to 

 answer this question, which involves more or less special conditions for 

 each of the different localities. Some of the alkaline feldspar syenites cut 

 both silicious and calcareous sediments. Their origin is made clearer by 

 an experiment by Schweig, who showed that if molten nephelite syenite is 

 saturated with silica, the resulting syntectic is approximately the average 

 pulaskite.^^ 



The possibility is recognized that juvenile as well as "resurgent'^ car- 

 bon dioxide (with other gases) may, in some cases, have accumulated in 

 subalkaline magmas, which have differentiated, giving alkaline poles 

 similar to those derived from syntectics with limestone. For the Cripple 

 Creek phonolitic rocks a special mechanism for the accumulation of 

 juvenile gases is suggested, but it is possible that the abundant carbon 

 dioxide of the mine workings there emanates from limestone sloped down 

 by the pre-Cambrian granites. Analogous difficulties are found in apply- 

 ing the general hj^pothesis to a few other districts, including Eed Hill, 

 New Hampshire ; the Great Eif t Valley of Africa ; some of the Brazilian 

 occurrences, etcetera. For these cases the petrologist needs more data 

 from the field geologist, especially as to the nature of the formations 

 making roofs and walls of the eruptive masses. Many papers, lacking in 

 this fundamental statement, are almost useless to the student of pe- 

 trogeny. 



6. Generalizing, the alkaline rocks may be regarded as differentiates 

 of syntectics. In most instances the foreign matter dissolved in the sub- 



10 M. Schweig: Neues Jahrbuch fiir Mlneralogie, etc. Beilage Band 17, 1903, p. 517. 



