224 PATAGONIAN EXPEDITIONS: GEOGRAPHY. 



the level of the sea, the material thrown down by the icebergs would be 

 remodelled by the tides and, augmented by that transported by the shore 

 ice of the rivers, would be thrown upon the beach as a great bed of coarse 

 shingle, in precisely the same manner in which we see it accumulating 

 to-day, over a region with a coast line of fully forty miles on the south side 

 of the Rio Gallegos, and over a less extensive area on the north side, as 

 seen in the photograph shown in Fig. i. 



The above theory as to the origin, age and mode of deposition of the 

 Shingle formation of Patagonia is the result of very careful and extensive 

 observations and studies made by the writer during his three years of 

 travel in that country. It is believed satisfactorily to explain all the more 

 important features in the case. The materials are finer and the beds 

 thinner as we recede from the Andes, because the distance from their 

 source of supply is greater, and they have, therefore, been subjected to 

 greater erosion during the process of transportation and deposition. They 

 cover alike the pampas, the valleys and the slopes of the latter, because 

 these were in existence previous to the last submergence. They and the 

 plains which they cover present a uniformly level surface, sloping very 

 gently to the east, because the elevation began to the westward and has 

 proceeded gradually but slowly eastward. The loftier basalt-capped table- 

 lands of the interior are not covered with shingle, because they were never 

 submerged, but existed as islands in the Pliocene sea. The necessary con- 

 ditions for the above theory as to the origin of the Shingle formation also 

 explain very nicely the formation of the terraced slopes of these plains. 

 The discussion of these, however, I must leave to the chapters on 

 the geology of the region, having already diverged too far from my 

 subject. 



The Transverse Valleys. — The surface of the Patagonian plains is dis- 

 sected by a series of deep, transverse valleys. These extend from the base 

 of the Andes eastward to the Atlantic coast. They are all true valleys of 

 erosion and most of them are still occupied by streams of greater or less 

 importance. Of these valleys only three are occupied by rivers that flow 

 throughout their entire course uninterruptedly during the whole year. 

 These are the Gallegos, the Santa Cruz, and the Chico of the Santa Cruz. 

 Of the remaining rivers the Desire, the St. Dennis, de Bajos, Sheuen, 

 Aubone, Coy and Chico of the Gallegos are perennial throughout their 

 upper courses, but more or less intermittent below, while in the valley of 



