IGNEOUS ROCKS OF TASMANIA. 293 
Sheffield, Bothwell, &c.; is hydrated at Perth; and tuff- 
aceous at Ringarooma, Leven, &c. Tachylyte of a beautiful 
blue colour occurs at Richmond and Nietta; the hue, no 
doubt, due to soda. 
Columnar jointing may be seen at Burnie and on the 
North Coast, east of the Tamar. No cinder cones have 
been found yet, and so far we have not been able absolutely 
to define any volcanic necks which served as conduits for 
the lava. The Tertiary olivine basalts appear to have been 
poured out at the end of the Paleogene epoch. They 
choked the river valleys at that time, covermg up stanni- 
ferous and auriferous gravels, and giving rise to deep leads. 
These basalts require more detailed study than they have 
yet received ; there would seem to be associated with them 
nepheline-bearing lavas, which cannot well be distinguished 
in the field from the flows of normal basalt. 
_ A Silurian basalt or melaphyre occurs at Zeehan, where 
it is known as the “ white rock.” It is there interbedded 
with the slates, and is generally vesicular—often tuffaceous. 
The vesicles are filled with calcite or a chlorite mineral. 
Associated with the nepheline-bearing basalt at the Bluff 
or Nut at Circular Head is a variety of basaltic rock, con- 
taining porphyritic hypersthene. The rock is only known 
by a slide, and its unexpected discovery is perplexing. 
ELZo.uite Rocks. 
A family bond unites the whole series of nepheline rocks, 
viz., an excess of alkalies. Accordingly, in nature, rocks 
with this characteristic will be found associated, and we shall 
not find them genetically connected with normal acidic and 
basic rocks. Hence I detach alkali-granites and alkali- 
syenites from the normal granites and syenites, and put 
them into the eleolite series. H. Rosenbusch treats them 
as resultants from the foyaitic magma, and I submit he is 
logically pledged to take the further step of giving them a 
place among the foyaitic rocks. Both alkali-granites and 
alkali-syenites occur in association with elezolite syenites, 
pass into them, and contain related dyke-rocks. Their Si 
per cent. is higher than in their normal equivalents, Mg 
and Ca less, but Na and K higher. Hence mineralogically 
the alkali-granites are characterised by an alkali-amphibole 
(riebeckite or arfvedsonite), egirine, soda orthoclase, micro- 
cline, microperthite, albite, and anorthoclase. The alkali 
granite lavas, comendite, and pantellerite contain similar 
soda minerals. For the same reason, alkali-syenites have 
to be transferred also. They are characterised by soda 
