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



allied to phonolites, and become phonolite by the addition 



of either of the felspathoids, leucite or nepheline. The 



roughness of ordinary trachytes is chai*acteristically 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 crow-ding of the porphyritic elements. This,. 



again, is not unknown among phonolitic trachytes and 



phonolites.* The presence of haiiyne is considered by 



some authors sufficient to remove a rock from the trachytes. 



to the i^honolytes, and they w^ould call some of the Port 



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



nosean (haiiyne) in the first British phonolite (Wolf Rock,. 



Cornwall) w^as discovered by Mr. Allport, in 1871, by 



means of the microscope, the same instrument has led to 



the discovery of haiiyne 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 there are structural differences dependent upon 



the physical conditions of consolidation. We are able,. 



m.ore or less plainly, to arrange the numerous varieties 



provisionalh", as follow^s : — 



« 

 ^^ . Soda Trachyte Group. 



Jyfusives — 



Haiiyne Trachyte, with loorphyritio haiiyne. 

 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. • 



Intriisives — 



Sanidine-augite-haiiyne Aplite. 



Sanidine-augite-biotite Aplite. 



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



„, , . Soda Sye>'Ite Group. 



Flutomcs — 



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



Alkali-augite-Syenite, with elaeoHte, (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, fiu'ther, 

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

 remarkaloly 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.. 

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



