BY H. I. JENSEN. 563 



have invaded the Hawkesbury, and are therefore Post-Triassic in 

 age. Messrs. Taylor & Mawson, who studied the petrology and 

 geology of this area,* regarded the intrusions as of late Cretaceous 

 age. Tn addition to trachytes and syenites of the alkaline facies 

 they found essexites, pic-rites, and basalts in the vicinity. The 

 trachytes and syenites were first erupted, the essexites, picrites, 

 and basalts coming later. The position of this mass is on the 

 outskirts of the Sydney coal-basin. Mr. Taylor has since dis- 

 covered outlying members of this group (green segirine trachytes) 

 at Goulburn. 



Almost north-west of the Mittagong alkaline area we have the 

 Canoblas Mountains near Orange. The chief igneous rocks 

 found here are old Palaeozoic (?) greenstones, alkaline trachytes of 

 Tertiary age, and more basic rocks. The igneous sequence is, 

 according to Mr. C. A. Sussmilch, F.G.S., the following: — 



1. Greenstone (hornblende diorite porphyry) and rhyolites, after 

 the eruption of which there was a very prolonged cessation of 

 activity lasting through all the Mesozoic periods. 



2. Trachytes which bear close resemblance lithologically, 

 mineralogically and chemically to those of the Nandewar Moun- 

 tains, N.S.W., and the Glass House Mountains, Queensland. 



3. Andesites and basalts, some of which are very porphyritic 

 in andesine, and seem to me to have the appearance of being 

 related to the alkaline series like the basalts of Exmouth in the 

 Warrumbungles. 



4. Later Middle Tertiary calcic basalts. 



As far as has been ascertained up to the present, we have no 

 evidenceof such complete series of alkaline rocks,with intermediate 

 types linking the acid to the basic, as in the Warrumbungles and 

 Nandewars. 



The trachytes intrude Palaeozoic rocks. No Mesozoic rocks 

 occur near them, so the exact age can only be guessed at. How- 

 ever, the crater-rings are so fresh that they cannot be older than 

 early Tertiary. 



* "Geology of Mittagong, " Journ. Proc. Roy. Soc. N.S.Wales. Vol.xxxvii. 



