270 Lt.-Oen. McMahon — Conversion of Chlorite. 



The second class of areas, which satisfy the conditions of oar 

 theory, are the deep ocean floors. Here we have depressed areas 

 surrounded by more elevated regions ; the surface layers being 

 maintained at a very low temperature, as determined by recent 

 soundings. These, in general, afford examples of the case in 

 ■which the passage from elevated to depressed areas is rapid. We 

 should therefore expect to find, along the borders of such areas, 

 evidence of volcanic phenomena. It is sufficient to quote, as illus- 

 trative examples, the vast ring of volcanoes w^hich surrounds the 

 Pacific Ocean. It seems also specially worthy of notice that Japan 

 is the scene of continually recurring earthquakes. Now it is on 

 the borders of the Pacific in the neighbourhood of Japan that we 

 find the most rapid passage from elevated to depressed areas, for 

 here is the deepest depression in the whole ocean. In consequence, 

 we have here a line of great weakness, and should expect earth- 

 quakes to be most frequent if in any way dependent upon the 

 above explained cause. As a last point of agreement with the 

 theory, we may notice Darwin's theory of Coral reefs as pointing 

 to the comparatively rapid sinking of the deep sea floors. 



lu conclusion, a few words may be said upon the necessary con- 

 sequences of the truth of this theory. It points unmistakably to 

 the permanence of continental and deep sea areas, and would 

 indicate that the tendency has always been to deepen and render 

 narrower the ocean basins. Perhaps, also, it points to a continual 

 tendency to render the globe more elliptical by increasing the 

 flattening at the Poles. It would also necessarily follow that great 

 elevations could only take place on the borders of deep oceans, or 

 in proximity to regions of prolonged cold. Lastly, that volcanic 

 phenomena may be expected to occur along the coast regions of 

 great oceans. 



It may be remarked, as a necessary deduction from the first of 

 the above statements, that the crust covering the areas with which 

 we have dealt must be extremely thin, and that, at the bottom of the 

 deep sea basins, we should expect to find the nearest possible approach 

 to the primitive crust. 



It is, of course, not assumed that this theory is applicable to all 

 cases of subsidence and elevation which, when local, are no doubt 

 due to much less potent forces, such as denudation and deposition. 



{To be continued.) 



VII. — On the alleged Conversion of Chlorite into Biotite by 



Contact Action. 



By Lieutenant- General C. A. McMahon, F.G.S. 



IHAYE to thank Dr. Callaway for the courtesy and good-humour 

 with which he has received my criticisms on his paper in the 

 December Number of the Geological Magazine. Would that all 

 controversial fencing were conducted with equally well-tipped foils. 

 My Eoman soldiers, in particular, have been received with a 

 kindness that reminds me of the friendly meetings of the British 



