348 Transactions.— Geology. 
larger boulders, and a lesser current flowing up on the inner side, thereby 
disturbing and heaving up the smaller shingle. It will be observed also 
that the inner side is characterized by small gravel and the outer by larger 
boulders; so that the bank itself may be said to be the centre of two 
actions, an outer and inner current. 
These two currents are performing the same work that a couple of 
navvies would do in building a breakwater of the same materials, the one 
heaving up the boulders on the one side and the other shovelling up the 
smaller material on the other, the only difference being a question of time 
as to the completion of the work. It has been suggested by some observers 
that there may have been at some early period a ridge of rock extending the 
whole distance, and that the present Arrow Rock at the entrance to our 
harbour is the termination of this supposed ridge, the other portion having 
been degraded and worn down, and the present bank resting on its base. 
This is a probable theory and may be correct, and certainly would go 
far in facilitating the building-up of the bank, it having got a solid founda- 
tion to rest upon, but I have never heard of its being verified by sinking or 
otherwise. I am however of opinion that the forces at present in operation, 
however inadequately I may have described them, are quite sufficient to 
form the bank without the help of this rocky base to rest upon. This is a 
subject it would be interesting to get proofs of, and probably some enquirer 
of this society who may have time and inclination to institute a research in 
this direction may communicate his views on some future occasion. 
I have said in the course of my remarks that this bank is chiefly com- 
posed of fragments from this syenitic bluff, and would hazard the opinion 
that, if this very hard crystalline rock had not been in the position we now 
find it, there would have been no Boulder Bank, or, had the bluff been sand- 
stone or any other rock of a softer and more friable material, the forces 
acting upon it would speedily have reduced it to sand or mud, which would 
not have had the resistance to form a breakwater—in fact, no other rock in 
the district would have been fitted for the work; and when the supply of 
fresh material from this source is exhausted, or ceases to be rolled down to 
supply the degraded waste going on in the bank below, the decay of the 
bank itself will take place—there are some appearances of this going on at 
the present time. A large bank of shingle has been and is being formed at 
the base of this syenitie dyke, the degraded fragments of which no longer 
drop into the sea to be rounded and carried down, but are left where they 
fall among this shingle, consequently the bank below is not receiving the 
same amount of fresh material necessary to its support in supplying the 
waste going on in consequence of the action of the sea, thereby a breach in 
the bank itself may sooner or later be the consequence and the sea find its 
