Speight. — Collection of Eocks from Norfolk Island. 331 



a greenish hornblende, minute zircons, and an irresoluble base full of grains 

 of iron-ore probably derived from mica. There is not sufficient evidence to 

 say definitely what was the nature of the parent rock — whether it was 

 volcanic or plutonic ; but the evidence that exists points perhaps in the 

 direction of the latter. If this is the case, it would afford some slight 

 evidence that the present volcanic mass of these islands was built up on a 

 foundation of plutonic rocks, and even if the fragments are not derived 

 from rocks of deep-seated origin there is undoubted evidence that the 

 islands are built up on a foundation of igneous rocks entirely different 

 from those now exposed everywhere on the surface. 



In general facies the rocks of the group appear to be more closely con- 

 nected with those of Lord Howe Island, and have no relationship to those 

 of the Kermadecs. It is impossible to give any idea of their age, except 

 that the extent to which nearly all are weathered and the diabasic cha- 

 racter of some Suggest an early Tertiary date for their extrusion. 



Art. XLI. — On a Shingle-spit in Lake Coleridge. 

 By R. Speight, M.Sc, F.G.S. 



[Read before the Philosophical Institute of Canterbury, 4th December, 1912.] 



Plate XVII. 



In a paper on the physiography of the Mount iVrrowsmith district* I drew 

 attention to two hooked spits in Lake Heron, whose formation, judging 

 from the conditions in which they occur, must be attributed entirely to 

 wave-action, and not to the dominant influence of a long-shore current. 

 I do not wish to infer from their occurrence that all spits are so formed, 

 for in the building-up of marine spits currents and tides do undoubtedly 

 play a very important part. Still, I think that the importance of waves 

 as spit-formers is not adequately appreciated, and if their efficiency in 

 lakes where currents do not occur can be thoroughly demonstrated the 

 proper value to be attached to wave-action in the sea may be perhaps 

 more accurately estimated. Some authorities, indeed, have insisted on the 

 importance of wave-action. One need only mention in this connection 

 that Gilbert, in his illuminatmg paper on the '' Topographic Features of 

 Lake-shores, "f notes that Cialdi was of opinion that waves were a dominant 

 factor in the formation of spits in the Mediterranean, although Gilbert 

 himself insists on currents being mainly responsible for them. In a 

 recent paper on the " Shingle-spit as a Plant-habitat, "J Professor Oliver, 

 as a result of observations on spits on the English coast, states that in 

 certain cases, at any rate, storm-waves do exert a marked effect on the 

 shape of hooked spits. In Professor Oliver's paper there are numerous 

 observations on minute points of interest in the structure of spits, to some 

 of which I shall have occasion to refer later, and which I shall be able to 

 confirm from observations in Lake Coleridge. 



* Trans. N.Z. Inst., vol. 43, p. 324, 1911. ^ 



t United States Geological Survey, Fifth Annual Report, 1883-^'.p 

 J New Phytologist, vol. 11, No. 3, March, 1912. / '• ' 



^(L,e. 



