Correspondence — Rev. 0. Fisher. 431 



that it is hopeless to look in this direction for any workable results. 

 And this being so, it is necessary to attack this problem, as many 

 other problems have had to be attacked, in a more roundabout way. 

 Mr. Fisher finds, if I am not mistaken, the statical conditions of 

 equilibrium in such solids when the forces exerted on them have 

 increased to such an extent that they are on the point of exhibiting 

 their plastic character. In doing so, however, he only considers the 

 forces acting on the solid vertically and in one horizontal direction. 

 It might perhaps lead to a useful result if he extended his method 

 to the consideration of the problem in three dimensions, as it seems 

 probable that another tension Q, corresponding to his tension P, but 

 in a direction perpendicular to that of P, must arise during contraction. 

 His present results agree in many respects with what we find in 

 nature. Thus he leads us to expect that no direct faults caused by 

 contraction will have less inclination to the horizon ^ than 45°. I 

 do not know of any of less inclination than this, but if any such 

 exist, the fact may most probably be accounted for on the supposition 

 that the whole strata, fault and all, have been subsequently turned 

 through an angle of dip sufficient to change the hade to its present 

 value. He also leads us to expect that series of crossed faults will 

 consist of two more or less parallel systems. A careful examination 

 of series of faults with such a guide to our enquiries as Mr. Fisher's 

 papers will afford, will be of infinitely more value to geology than 

 any amount of random onslaught by careless critics. 



A. F. Griffith, M.A. 

 Sandridge, St. Albans, August 12th, 1884. 



THE PERMANENCE OP OCEAN BASINS. 



Sir, — Mr. Mellard Eeade has drawn attention to the discovery 

 that South Georgia is not a volcanic island, but is composed, 

 of clay- slate : ^ and argues from this fact against the theory of the 

 Permanence of Oceanic and Continental Areas. He very fairly 

 remarks that, if islands like New Zealand are largely composed of 

 sedimentary rocks, they are said not to be oceanic, and that in 

 arguing from the position that all truly oceanic islands are volcanic, 

 the advocates of the theory arbitrarily exclude the non-volcanio 

 from the category of oceanic islands. 



There is, however, something to be said on the other side. The 

 non-volcanic islands mentioned by Darwin in his "Coral Islands" 

 are New Caledonia, and the Comoro and Seychelles. New Caledonia 

 seems to be a link in the chain which connects New Zealand with 

 New Guinea, and lies in the course of the great volcanic band 

 which stretches through Java to New Zealand. The Comoro islands 

 are too near Africa to be called oceanic ; and the Seychelles appear 

 to be on the axis of Madagascar, and may well be connected with it. 



^ There seems to be much uncertainty among geologists as to the use of the word 

 "hade." Among miners it appears to be measured always from the vertical, and it 

 ■would perhaps be well for us to assimilate our use of the word to theirs, as we 

 borrowed the term from them. 



2 Geol. Mag. May, 1884. 



