334 Transactions. 



did really occur in former times, before this part of the lake was made 

 shallower by detrital accumulations. 



Form op the Spit. 



The form of this spit is by no means as perfect as those in Lake Heron. 

 It is modified to some extent by the doubling of the loop, but both show 

 a decided convexity to the lake. Gilbert insists that lake-spits usually 

 exhibit a concavity towards the open water, but this feature is absent ia 

 all lake-spits that I have seen. If spits are formed in the way I suggest, 

 then they must necessarily, in their initial states at least, be convex to the 

 open water, as the evolute of a number of wave-fronts will have its out- 

 ward curve turned in that direction. There is, however, a process in 

 operation in some spits, and certainly in those iu Lake Coleridge, which will 

 turn a convex curve into a concave one. All along the distal portion of 

 this spit there are signs that it is being moved bodily backward towards the 

 shore by the action of the waves. During strong winds, and at times when 

 the level of the lake is above normal, the beach is driven landwards and 

 buUt up in the form of a levee, and this in spite of the stability which has 

 been given to it by the covering of scrub and other vegetation. This spit 

 is covered with low forest containing the following trees : Manuka, kowhai, 

 the cabbage-tree, and the scrub-formers such as Discaria and Svttonia, 

 together with flax and other swamp-plants. These are bemg rapidly up- 

 rooted along the outer margin, and the shingle beach is piled among those 

 which are still standing. Growing flax which has been dislodged can also 

 be seen beneath the water of the lake. During especially high winds the 

 waves break over this levee, and form small washouts on the inner side, which 

 continue the landward movement of the spit. This process is continued, 

 though in a slightly less intense form, by the percolation of water through 

 the bank when the waves are not so high. Small washouts are thus formed 

 in a manner analogous to that described by Professor Oliver in the case of 

 the Chesil Bank, on page 88 of his paper. The resemblance seems to be 

 exact, except that the phenomena in Lake Coleridge are on a smaller scale. 

 However, in both cases the net result is to move the spit landwards. I 

 am not fully aware of all the circumstances of those spits referred to as 

 concave by Gilbert, but I would suggest that this process will make even 

 a convex spit sag landwards eventually. The movement may continue 

 till the proximal portion of the spit becomes a true shore beach. This 

 has occurred in the Lake Coleridge spit. It is also true in one of the most 

 notable spits in New Zealand — viz., that near Nelson, locally called the 

 Boulder-bank. This is some thirteen miles in length, and where tied on 

 to the land is distinctly concave ; but its distal portion is convex, and its end 

 has the typical form of a true incurved spit. 



In his paper Professor Oliver evidently accepts Gilbert's statement that 

 lake-spits are concave to the open water, and, as he notes that marine spits 

 are frequently convex, he concludes that there is some inherent difference 

 between the formation of lake and sea spits. It appears to me that this 

 distinction is not based on a solid foundation. Some lake-formed spits are 

 undoubtedly convex, and some marine ones are concave. The difference 

 in form appears to be due to the relative importance of wave-action and 

 current-action during the process of building. If the former is predomi- 

 nant, then in their initial stages, at all events, spits will tend to be convex. 

 They may become concave owing to sagging shorewards at a later date, a 

 result accelerated by a slight depression of the land or rise in the water of 



