BY W. H. TWELVETREES AND W. F. PETTERD. 7 



allied to phonolites, and l^ecome plionolite by the addition 

 of either of the felspathoids, leucite or nepheline. The 

 roughness of ordinary trachytes is characteristically absent, 

 and the disposition of felspar crystals in layers is a 

 marked feature, imparting a certain degree of fissility to 

 the rock. The rock has evidently possessed in its molten 

 state exceptional viscosity, which has impeded free flow 

 and caused* crowding of the porphyritic elements. This, 

 again, is not unknown among phonolitic trachytes and 

 phonolites.* The presence of haliyne is considered by 

 some authors sufficient to remove a rock from the trachytes 

 to the phonolytes, and they would call some of the Port 

 Cygnet rocks phonolytes. It is interesting to note that, as 

 nosean (haliyne) in the first British plionolite (Wolf Rock^ 

 Cornwall) was discovered by Mr. Allport, in 1871, by 

 means of the microscope, the same instrument has led to 

 the discovery of haliyne and phonolitic trachytes at Port 

 Cygnet. 



Though the eruptive rocks of this province are evidently 

 products of one and the same magma, yet different flows 

 at different times show varieties of mineral composition in 

 all probability characteristic of each eruption, in addition 

 to which tliere are structural differences dependent upon 

 the physical conditions of consolidation. We are able, 

 more or less plainly, to arrange the numerous varieties 

 provisionally, as follows : — 



TT^ • Soda Trachyte Group. 



Haliyne Trachyte, with porphyritic haliyne. 

 Aegirine Trachyte, with aegirine needles and aegirine-augite. 

 Melanite Trachyte, with abundant melanite-garnet. 

 Trachyte, with green sodic augite. 

 These comprise various types, described in detail further on. 



T . . Soda Aplite Group. 



Intrusive s — 



Sanidine-augite-haiiyne Aplite. 



Sanidine-augite-biotite Aplite. 



Malchite or dyke-Diorite, (an aberrant member). 



Plutonics- ^^^/^ Syenite Group. 



Alkali-augite-Syenite, with micro-perthite and analcime. 

 Alkali-augite-Syenite, with elaeolite, (Little Oyster Cove). 



* '• We find that acid lavas are very decidedly less mobile than basic 

 ones, and so flow less readily and to smaller distances ; and, further, 

 that certain intermediate lavas, rich in alumina and potash, are 

 remarkably viscous, as is illustrated by the peculiar dome-like forms 

 assumed by some trachytic and phonolitic eruptions." Daubree 

 "believes that some trachytic domes must have been erupted in a nearly 

 solid, not even pasty, condition." Nat. Hist, of Igneous Rocks. A. 

 Barker. Sc. Prog-. Vol. VII. pp. 204-6. 



