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 Bock 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 syenitic 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 



