6 Scientific Proceedings, Royal Dublin Society. 



erected during the period of the first settlement of the islands in 

 1796 — that they are now in a sinking condition. 



Some of the trunks of trees which he observed had their roots 

 attached to the soil, and were still in situ ; many of them belonged 

 to species which do not occur in the outer belt of mangroves, but 

 in the next succeeding zone inside it, or in a zone at the foot of 

 cliffs, in which positions there is no outer belt of mangrove swamps 

 owing to the steepness of the shore. 



On the one hand, the fact that the above-mentioned straits have 

 become unnavigable may be due to either ordinary or extraordinary 

 silting up. By extraordinary, I mean wholesale landslips, such as 

 those in Patagonia, and therefore their condition might be coin- 

 cident with a general subsidence. On the other hand, mere en- 

 croachments of the sea may produce appearances which would be 

 mistaken for actual subsidence. This last may be the explanation 

 in the case of the above-mentioned buildings, for instance. 



Now, without attempting to discuss the conflicting evidence in 

 this particular case, as further examination of the islands, espe- 

 cially with reference to the views put forward below, should, I 

 think, first be made, I pass to the consideration of the general 

 question as to the manner in which such apparently conflicting 

 evidence may be reconciled. 



The subject has already been dealt with in a Paper by Dr. Cop- 

 pinger, which was read before the Geological Society of London 

 last year.^ The phenomena which he describes were observed in 

 Western Patagonia, where the soil-cap slides downwards over 

 sloping surfaces of rock, carrying with it not only the trees, ferns, 

 and mosses on its surface, but also a moraine profonde of rocks, 

 stones, and trunks of trees, with which the valleys and lakes 

 become filled up. 



Anticipating that subsidence might be suggested to account 

 for some of the phenomena, he points out that the existence of 

 raised beaches, and the traces in the rocks of stone-boring moUusca 

 above present sea level, prove, on the contrary, that elevation has 

 taken place. Eemoval by water of the lighter portions of the 

 material thus brought down leaves a remnant of blocks, which are 

 often perched on one another in a similar manner to those which 



1 Quarterly Journal of the Geological Society. yoI. xxxvii., 1881, p. 245. 



