220 Scl.enlific Proceedings. Rot/al Dublin Societij. 



X. 



Summary of Conclusions. 



We are now in a position to make a general survey of the work we have 

 gone through, and, as we saw at first, the evidence at our disposal leads us to 

 believe that, after a long period of dry climate with high westerly winds, 

 probably in later tertiary times, the country to the north of the lower 

 reaches of the Gallegos river became so levelled off that not a hillock 

 practically could be found for hundreds of miles. The surface was as level 

 as the sea, and if any inclination existed it was a slight fall from the moun- 

 tains in the west towards the Atlantic. During this time the land in 

 question was slowly rising, but not at a uniform rate, since we find three 

 well-marked steps from north to south, indicating three old coast-lines. The 

 pampas between Gallegos and Coyle towards the end of this period had 

 partly subsided again, and portion of the land at Cape Tairweather was 

 slightly below the sea-level. When things had reached this point, a change 

 occurred, the climate suddenly began to get colder, and the ice, which was 

 heretofore limited to the higher levels of the mountains, began to descend 

 into the plains. As the ice descended it caused considerable smashing up of 

 the rocks, and it carried a huge quantity of broken debris with it. All this 

 time the climate was getting colder and colder, so that the spring floods 

 were not excessive, and consequently had no great eroding power. The 

 huge quantities of broken debris carried down by this ice-sheet became 

 deposited partly under it and partly along its termination; this limit is 

 shown by a line marked on the map. The spring and summer waters, melt- 

 ing away from this great ice-mass, formed slowly running and expanded 

 streams, and, as the country was one huge slightly inclined plain, these 

 streams could not follow any definite valley, but ran broadcast over the 

 country. They carried with them much of the finer sand and shingle which 

 the ice had brought to its limits, but left all the large blocks behind. This 

 condition of affairs lasted until the pampas were covered with a layer of 

 shingle, which extended right to the Atlantic coast, and probably a long 

 distance into the sea. When the pampas had become thus completely 

 covered, a change occurred once more in tlie climate ; the seasons began to 

 get warmer, and each summer a greater quantity of ice and snow melted 

 than was formed during the winter. 



The result of this was that the spring and summer floods increased, and, 

 in place of depositing sand and gravel, they gave rise to huge torrential 

 rivers, with very considerable erosive powers. By the time the ice had all 

 melted away, and the country had settled down once more to a condition of 



