BY H. I. JENSEN. 503 



possible, the age has been found to be near Eocene. Where such 

 -a determination has not been found possible the alkaline masses 

 intrude very old Archaean or Palaeozoic strata. 



In this connection it is apropos to mention that the erosion of 

 about 400 feet of Mesozoic sediments in the district of the Glass 

 House Mountains, Queensland, would leave the plugs and stoeks 

 of alkaline rock surrounded by the Palaeozoic and Archaean com- 

 plex which underlies the Trias-Jura rocks of the district, and it 

 would be impossible for any geologist to locate their age any nearer 

 than as Post-Carboniferous or, in some cases, Post- Archaean. 



In very many cases — possibly that of Mount Macedon is one — 

 the Mesozoic sediments originally invaded have been denuded 

 away. 



The Association of Alkaline Rocks with the Archaean 



Complex. 



From the foregoing discussion it is gathered that alkaline rocks 

 are frequently found intruding very old rocks in such a way that 

 their age cannot be determined, and in all other cases rocks repre- 

 senting the oldest Palaeozoic and the Archean can be found at no 

 great distance from the alkaline areas. 



. Instances of this fact have already been given, and many more 

 will be discussed later when each alkaline region is dealt with in 

 more detail. In the United States instances abound; in Africa 

 we have only to point to the cases of Abyssinia and Algeria. In 

 Asia we have the same associations in Siberia and Arabia; in 

 Brazil we have another notable example. For Australia we have 

 only to mention the fact that the Glass House Mountains are 

 only a few miles from the extremely metamorphic schist-region 

 of the D'Aguilar Range. The Nandewars have the late Palaeozoic 

 basin of New England to the east of them, while to the west 

 there is the region of plains, under whose black soils and red 

 loams we have an ancient complex or a thin capping of Mesozoic 

 sandstone overlying this complex. The same holds true for the 

 Warrumbungles. The Canobolas rise directly out of the region 

 of metamorphic Palaeozoic rocks; and their recent age is only to 



