chap. viii. Uniform Height of Terraces. 213 



modelled into their present forms, has been singularly 

 uniform. 



These plains or great terraces, of which three and 

 four often rise like steps one behind the other, are 

 formed by the denudation of the old Patagonian 

 tertiary beds, and by the deposition on their surfaces of 

 a mass of well-rounded gravel, varying, near the coast, 

 from ten to thirty-five feet in thickness, but increasing 

 in thickness towards the interior. The gravel is often 

 capped by a thin irregular bed of sandy earth. The 

 plains slope up, though seldom sensibly to the eye, 

 from the summit-edge of one escarpment to the foot of 

 the next highest one. Within a distance of 150 miles, 

 between Santa Cruz to Port Desire, where the plains 

 are particularly well developed, there are at least seven 

 stages or steps, one above the other. On the three 

 lower ones, namely, those of 100 feet, 250 feet, and 

 350 feet in height, existing littoral shells are abundantly 

 strewed, either on the surface, or partially embedded in 

 the superficial sandy earth. By whatever action these 

 three lower plains have been modelled, so undoubtedly 

 have all the higher ones, up to a height of 950 feet at 

 S. Julian, and of 1,200 feet (by estimation) along St. 

 George's Bay. I think it will not be disputed, con- 

 sidering the presence of the upraised marine shells, that 

 the sea has been the active power during stages of some 

 kind in the elevatory process. 



We will now briefly consider this subject : if we 

 look at the existing coast-line, the evidence of the 

 great denuding power of the sea is very distinct ; for, 

 from Cape St. Diego, in lat. 54° 30' to the month of 

 the Rio Negro, in lat. 31° (a length of more than 800 

 miles), the shore is formed, with singularly few excep- 

 tions, of bold and naked cliffs : in many places the 

 cliffs are high ; thus, south of the Santa Cruz, they are 



