224 Gravel Formation of Patagonia, pake n, 



ness over tlie whole area of 630 by 200 miles, at fifty 

 feet! 



The transportal and origin of this vast bed of pebbles 

 is an interesting problem. From the manner in which 

 they cap the step-formed plains, worn by the sea within 

 the period of existing shells, their deposition, at least 

 on the plains up to a height of 400 feet, must have 

 been a recent geological event. From the form of the 

 continent, we may feel sure that they have come from 

 the westward, probably, in chief part from the Cordillera, 

 but, perhaps, partly from unknown rocky riclges in the 

 central districts of Patagonia. That the pebbles have 

 not been transported by rivers, from the interior towards 

 the coast, we may conclude from the fewness and small- 

 ness of the streams of Patagonia : moreover, in the case 

 of the one great and rapid river of Santa Cruz, we have 

 good evidence that its transporting power is very trifling. 

 This river is from 200 to 300 yards in width, about 

 seventeen feet deep in its middle, and runs with a sin- 

 gular degree of uniformity five knots an hour, with no 

 lakes and scarcely any still reaches : nevertheless, to 

 give one instance of its small transporting power, upon 

 careful examination, pebbles of compact basalt could 

 not be found in the bed of the river at a greater distance 

 than ten miles below the point where the stream rushes 

 over the debris of the great basaltic cliffs forming its 

 shore : fragments of the cellular varieties have been 

 washed down twice or thrice as far. That the pebbles 

 in Central and Northern Patagonia have not been trans- 

 ported by ice- agency, as seems to have been the case to 

 a considerable extent farther south, and likewise in the 

 northern hemisphere, we may conclude, from the absence 

 of all angular fragments in the gravel, and from the 

 complete contrast in many other respects between the 

 shingle and neighbouring boulder formation. 



