94 PETROLOGY OF THE ALKALINE ROCKS 



simultaneously with mine, ascribes most igneous magmas to assimilation of crustal 

 rocks by an olivine basalt magma. According to these views olivine basalt is the 

 primary magma. Through assimilation of alkaline sedimentaries of the earth's crust 

 the kenyte mother magma has been formed. This, again, may produce olivine basalt 

 by differentiation. 



In a more recent paper, entitled " The Origin of Alkaline Rocks," * Professor Daly 

 supposes these rocks to be differentiates from a normal basalt magma following inter- 

 action with limestones at a depth. This hypothesis seems a very probable one in 

 theory, chemically, physically, and mineralogically sound, but unfortunately the field 

 evidence in Australia lends it so far only little support. Yet I think it highly probable 

 that many alkaline and subalkaline bodies have originated in this way, and others in 

 the way I have suggested. 



G. T. Prior has assigned the term " Atlantic type " to all rocks which might under 

 the Rosenbusch classification be considered differentiates of a foyaitic magma. For 

 reasons given in my paper on "The Distribution, Origin ... of Alkaline Rocks," I 

 prefer to call the type to which they belong " Katepeiric." 



The lavas of Ross Island are in this report treated under the following headings : 



1. Trachyte. 



2. Acid Kenyte. 



3. Intermediate to Basic Kenytes. 



4. Trachydolerites (porphyritic basalts with alkaline affinities). 



5. Leucitophyres, Tephrites, and Basanites. 



6. Basalts without Olivine. 



7. Olivine Basalts. 



8. Limburgites. Magma Basalts. 



9. Magnetic Basalts. 



Generally speaking, this order of treatment is one of decreasing acidity and 

 alkalinity, but a strictly natural arrangement is impossible, for the following reasons : 



(a) The trachydolerite group passes insensibly into that of the limburgites and 

 vice versa. 



(b) Some of the tephrite- basanite group are more basic than the basalts with or 

 without olivine. 



(c) The limburgites have stronger affinities with the trachydolerites and basanites 

 than with the basalts. 



(d) The magnetite basalts sometimes have affinities with the kenytes. 



T he following schematic representation gives some idea of the relationships of these 

 rocks as far as penological investigation can decide (vide p. 95). 



1. THE TRACHYTES 



Localities. — Most of the trachytes which were collected came from a parasitic cone 

 to which the name of Mount Cis has been given. Other trachytes were collected at 

 Observation Hill, Hut Point, Mount Bird, Turk's Head, and a tuff cone on Ross Island. 

 In miscellaneous collections of erratics from Cape Royds and other parts several 

 trachytes were also contained. 



(a) Mount Cis Trachytes. — Those from Mount Cis were the most typical trachytic 

 rocks collected. They belong to the group of phonolitic trachytes. The dominant 

 variety is a dark grey or greenish-black vesicular rock with a glistening sheen, 

 at once suggesting the presence of an abundance of sanidine or anorthoclase. The 



* Bull. Geol. Soc. of America, vol. xxi, May 1910. 



