TRACHTDOLEEITE IX TASMANIA. 135 



The term trachydolerite here has not the original sense in 

 which it was used in 1841 by Abich, who understood by it 

 a basic trachyte or a rock intermediate between trachyte 

 and basalt. Its alkali percentage is high, viz. — 6 to 11%, 

 against the usual 3% or 4% in ordinary basalt. The 

 variety of constitution points strongly to essexite parentage, 

 and as essexite occurs physically connected with elaeolite 

 syenite, so trachydolerite is associated with alkali trachytes, 

 phonolites, tephrites, &c. Its sp. gr. is 2 • 86. 



Professor F. Loewinson-Lessing differs from this Rosen- 

 buschian view, and ranges trachydolerite among monzonitic 

 magmas, and not among essexitic ones.^ He treats it as 

 equivalent to ciminite (Washington), a passage rock be- 

 tween basalt and trachyte, and would suppress ciminite in 

 its favor or in favor of " trachyte basalt." It is, then, 

 considered the effusive equivalent of gabbro syenite or 

 monzonite. But, as there is reason to regard the basic 

 syenite known as monzonite as being not so much a passage 

 rock between syenite and gabbro, as between normal syenite 

 and alkali syenite, so trachydolerite must be looked upon as 

 intermediate, not between normal basalt and trachyte, but 

 between tephrite and alkali trachyte. The occasional 

 presence in it of haiiyne, sodalite, barkevikitic hornblende, 

 anorthoclase, leucite, and aegirine points to the alkali 

 magma. 



It is possible that some of our other Tertiary basalts may, 

 on closer examination, prove to be trachydoleritic ; and the 

 eventual discovery of the parent plutonic rock, essexite, is 

 probable. 



* Kritisclie Beitrage zur Sysieiiiatik der Eruptivgesteine : Tschermak's 

 min. n, petrogr. Mittheilungen xix. 1900. iv. p. 303. 



