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B. W. COPPINGER ON SOILCAP-MOTION. 



25. On Soilcap-Motion. By E. W. Coppingek, Esq., M.D. 

 (Eead March 23, 1881.) 



(Communicated by the President.) 



I wish to call attention briefly to a phenomenon which, so far as I 

 am aware, exists to an unparalleled degree about the shores of 

 Western Patagonia, and whose presence there is in a great measure 

 due to the exceptionally wet nature of the climate. I allude to a 

 slippage of the soilcap, which is, I believe, continually taking place 

 over the basement rock wherever the latter presents a moderately 

 inclined surface. Some of the effects of this soilcap-motion are apt 

 to be confounded with those due to glacial action ; for the soilcap 

 takes with it in its progress not only its clothing of trees, ferns, 

 and mosses, but also a " moraine profonde " of rocks, stones, stems 

 of dead trees, peat and mud, whereby the hills of this region are 

 being denuded, and the valleys, lakes, and channels, gradually 

 filled up. 



On arriving at the Patagonian archipelago my attention was 

 directed to this subject on noticing that the lower branches of 

 trees fringing the sea-shore were in many places withering from 

 immersion in the salt water, and that in some cases entire trees 

 had perished prematurely from their roots becoming entirely sub- 

 merged. On looking more closely I observed that the sodden 

 snags of dead timber, mingled with stones, were often to be seen 

 at the bottom of the inshore waters, and that the beds of fresh- 

 water lakes were plentifully strewn with similar fragments of wood, 

 the remains of forests prematurely destroyed. As the soilcap, by 

 its sliding motion, reaches the water, the soluble portions are 

 removed ; and just as stones and boulders are often seen deposited 

 in grotesque situations by a melting iceberg or a receding glacier, 

 so are the phenomena of "perched rocks" to be here observed, 

 although, in the class of cases to which I refer, due to a totally 

 different cause. These facts are all the more interesting from their 

 occurring in a region where the effects of old glacial action are to 

 be seen in a marked degree. Planings, scorings, striae, and " roches 

 moutonnees " may almost invariably be found wherever the rock is 

 sufficiently capable of resisting the disintegrating influence of the 

 weather to retain these impressions. Thus they are nowhere to 

 be seen on the coarse-grained friable syenite, which is the com- 

 mon rock-formation of the district : but where this rock is 

 intersected by dykes of the more durable greenstone, the above- 

 mentioned signs of former glacial action may be seen well deve- 

 loped. There are therefore in this region ample opportunities of 

 comparing and differentiating phenomena which have resulted from 

 " glacial action " and those which are due to " soilcap-motion " — a 

 force now in active operation. 



I may here observe that we did not see any glaciers worthy 



