BY H. I. JENSEN. 561 



alkaline nature, were partially incorporated in the magma. 

 Before assimilation had progressed far, a portion of the mass 

 broke through to the surface, forming the Blow Hole Flow, and 

 the vent thus formed gave after this event more and more alka- 

 line lavas produced by assimilation at a depth. Occasionally 

 more calcic varieties burst through. 



Later on another vent was formed at the Cambewarra end of 

 the reservoir, and from it flowed at first alkaline lavas formed 

 by the assimilation-process mentioned. 



From other fissures pasty alkaline magmas were expelled from 

 time to time from the surface of the magma-reservoir, and were 

 forced into the strata above in the form of sills. These injec- 

 tions gave rise to the tinguaite and nepheline syenite mentioned 

 above. 



Lastly, the mixed magmas having cooled, basic magmas were 

 drawn from the more deep-seated and still molten portions of the 

 reservoir. These eruptions were probably contemporaneous with 

 the Post-Triassic basalts of Sydney which are analcite-bearing, 

 and which frequently contain coarse fragments of peridotite as 

 at Pennant Hills (Dundas Quarry), and of trachyte or syenite 

 (as at Prospect). 



In connection with the occurrence of analcite in the diabase of 

 the Robertson Flow and of Prospect, and the theory that these 

 lavas were emitted from a magmatic reservoir in which great 

 assimilation of sedimentary strata had gone on, one might briefly 

 refer to a paper on " The Plumose Diabase and Pelagonite from 

 the Holyoake Trap Sheet," by B. K. Emerson.* It is here con- 

 tended that the formation of analcite is due to the drawing into 

 the magma by vortex-motions in it of a mass of mud. 



The Tinguaites of Barrigan. — These rocks are 

 referred to in Mr. Carne's monograph on the Kerosene Shales 

 and Torbanites of New South Wales. They form a number of 

 rounded mountains near Barrigan, and their age is definitely 

 later than Permo-Carboniferous. These rocks are laccolitic and 



* Bull. Geol. Soc. Amer. Vol.xvi. 



