48 GEOLOGICAL EXCURSION TO PORT CYGNET. 



Family 5. — Theralite. 



(c) Effusive representatives : — 



1. Nepheliriite : on the Shannon Tier (nepheline + augite). 



2. Limburgite : a dyke on the Emu Bay railway, near 



Burnie (olivine + augite). 



The alkali rocks form one of the two primary divisions of the 

 eruptives. One division is that comprising granites, syenites, 

 diorites, gabbro, and the ultra basics : the other comprises 

 the filkali eruptives. The latter, though not so universally abun- 

 dant as the former, illustrate a remarkable aptitude for differentia- 

 tion in the alkaline magma : hence numerous variations from 

 typical forms and increasing additions to the nomenclature. The 

 occurrences of Tasmanian rocks belonging to this division are con- 

 sequently of distinct interest to both the petrographer and the 

 geologist. 



