Fenton — Physiography and Glacial Geoloyy of S. Patagonia. I'd! 



would account for its undulatoiy surface, and, as we shall see later, its lack 

 of teiTaces. Is'evertheless, although the levels differ to the north and south 

 of this line, the superficial formations would seem to be the same, namely, 

 typical shingle overlying tertiary rock. 



Now, when we leave the Eio Chico, and travel southward towards the 

 Straits of Magellan, after a few miles we notice a new featui'e in addition to 

 the shingle beginning to ajipear ; this is an occasional great erratic boulder.' 

 At first such boulders are few, but after a league or two they become more 

 numerous. Some of these erratics are found on the surface, some are half 

 way out, whereas some are practically buried, with only their heads showing. 

 Now, these erratics have the same compositions as the pebbles in the 

 shingle ; even amongst the lava sheets, practically none of them consist of 

 lava, and there is a complete gradation in size from the smallest pebble up to 

 the largest block ; consequently, one is led to believe that they are part and 

 parcel of the same formation as the shingle, and have come from the same 

 locality. The erratics extend right to the Atlantic coast, ten miles south of 

 the mouth of the Gallegos river, and, like the sliingle, they are found equally 

 well marked on the highest hills and in the lowest valleys. A line drawn 

 through southern Patagonia on the Argentine side from a point on the 

 Atlantic coast, about twelve miles north of the fifty-second parallel of lati- 

 tude due west, to a point about twelve miles to the eastward of the 

 seventy-first west meridian, and then curving slowly to the north, following 

 finally up along and parallel to the Cordilleras, would more or less mark the 

 limit of the large erratic boulders. This line is not quite straight, and there 

 are minor curves in it, which, however, are of no great extent. I have traced 

 it as well as I can on the map. 



On going further to the north, along tlie chain of the Andes, one finds 

 erratics extending a certain distance into tlie plains, but not to anything 

 like the same distance as in the region which I have indicated ; in fact, this 

 very abrupt extension outwards towards the Atlantic of the large erratic 

 boulders to the south of the Gallegos river is a very remarkable fact, 

 especially when we consider that the mountains to the west of this district 

 are insignificant as regards height in comparison with those to the north 

 and south. 



Now, although the pampas to the south of the Gallegos river, near its 

 mouth, belong to a lower level than those to the north, yet further to the 

 west they both merge with one another, so that at a point sixty miles from 

 the coast all the pampas, both north and south, are on the same level ; here 



' Compare B. Hatcher, American Journal of Science, ser. 4, vol. iv, p. 348, 1897. 



