42 GEOLOGICAL EXCUKSION TO PORT CYGNET. 



picturesque. Wooded heights ascend from the water's edge 

 on each side, and large fi'uit orchards diversify the aspect of 

 the slopes. The view eastwards is shut in by the high range 

 in the background, a ridge of Mesozoic diabase (dolerite), 

 the holocrystalline plagioclase-augite rock, which H. M. 

 Johnston shows, is prolonged southwards from Mt. Wellington 

 and apparently forms the axis of the peninsula which divides 

 the w^aters of Port Cygnet from the Channel. This high 

 ridge is flanked by Permo-Cai'bonifei'oas sandstone and miid- 

 stone on either side, and, according to V. J. Ernst, crosses 

 the zone of alkali rocks in their N. E.-S.W. course. West- 

 wards the same Permo- Carboniferous beds are found, also 

 broken through by the elseolite syenites, tinguaites and allied 

 rocks, which the members of the Section now hastened to 

 examine in s?fu. 



At the extreme head of the arm the water is shallow, and 

 old residents say that it has receded considerably in recent 

 years. The shores are flat, and no good exposures of rock 

 are visible. The first outcrop of the alkali rocks is seen on 

 the shore, between the two jetties, but a more striking 

 development occurs at, and immediately south of, the 

 Regatta ground. This point, the termination of a projecting 

 headland a few hundred feet wide, consists of oJaeolite-syenite 

 with varieties of alkali and alkali-quartz, syenite passing at 

 each margin into darker varieties, which have been determined 

 by Professor H. Rosenbusch, to whom samples were submitted 

 by the Mines Department, as jacupirangite, essexite, and 

 nephelinite (or monchiquite). Generally speaking, there 

 are no sharp divisions betw^een the central mass of light- 

 coloured syenite and the dark marginal rocks, though well- 

 detined bands and veins of the lighter rock traverse the 

 other, sometimes in such profusion as to form a meshwork. 

 At other times the most gradual variation is seen from one 

 to the other, the lighter variety growing darker by imper- 

 ceptible stages. The darker rocks also vary considerably, 

 both in texture and colour. It is impossible to resist the 

 conclusion that we are here in the presence of an example of 

 magmatic differentiation. The leucocratic centi-e is elfeolite- 

 syenite, and the dark marginal rocks are diiferentiated 

 products. Of the latter, the mica nephelinite (containing 

 large crystals of biotite) has only been found as scattered 

 stones, but a little excavation work would probably reveal 

 the bed-rock. The locality is between the Regatta Box and 

 the Point. Professor Rosenbusch says this rock is nepheli- 

 nite with the habit of a monchiquite, to some extent actually 

 a monchiquite with a groundmass of nepheline, instead of 

 analcime or glass. Monchiquite is usually considered to be 



