chap. viii. Valley, of Santa Cruz. 207 



I co aid not perceive any trace of a similar deposi- 

 tion on the pebbles now thrown up by the river, and 

 therefore I do not think that terrace D was river- 

 formed. As the terrace E generally stands about 

 twenty feet above the bed of the river, my first impres- 

 sion was to doubt whether even this lowest one could 

 have been so formed ; but it should always be borne in 

 mind, that the horizontal upheaval of a district, by in- 

 creasing the total descent of the streams, will always 

 tend to increase, first near the sea-coast and then 

 farther and farther up the valley, their corroding and 

 deepening powers : so that an alluvial plain, formed 

 almost on a level with a stream, will, after an elevation 

 of this kind, in time be cut through, and left standing 

 at a height never again to be reached by the water. 

 With respect to the three upper terraces of the S. Cruz, 

 I think there can be no doubt, that they were modelled 

 by the sea, when the valley was occupied by a strait, in 

 the same manner (hereafter to be discussed), as the 

 greater, step-formed, shell-strewed plains along the 

 coast of Patagonia. 



To return to the shores of the Atlantic the 840 

 feet plain, at the mouth of the Santa Cruz, is seen ex- 

 tending horizontally far to the south ; and I am in- 

 formed by the Officers of the Survey, that bending round 

 the head of Coy Inlet (sixty-five miles southward), it 

 trends inland. Outliers of apparently the same height 

 are seen forty miles farther south, inland of the river 

 Gallegos ; and a plain comes down to Cape Gregory 

 (thirty-five miles southward), in the Strait of Magellan, 

 which was estimated at between 800 and 1,000 feet 

 in height, and which, rising towards the interior, is 

 capped by the boulder formation. South of the Strait 

 of Magellan, there are large outlying masses of appa- 

 rently the same great table-land, extending at intervals 



