chap. viii. Gravel Formation of Patagonia. 2 2 1 



underlying tertiary formations. The pebbles are fre- 

 quently whitewashed and even cemented together by 

 a peculiar, white, friable, aluminous, fusible, substance, 

 which I believe is decomposed feldspar. At Port Desire, 

 the gravel rested sometimes on the basal formation of 

 porphyry, and sometimes on the upper or the lower 

 denuded tertiary strata. It is remarkable that most 

 of the porphyritic pebbles differ from those varieties of 

 porphyry which occur here abundantly in situ. The 

 peculiar gallstone-yellow variety was common, but less 

 numerous than at Port S. Julian, where it formed 

 nearly one-third of the mass of gravel ; the remaining 

 part there consisting of pale gray and greenish por- 

 phyries with many crystals of feldspar. At Port S. 

 Julian, I ascended one of the flat-topped hills, the de- 

 nuded remnant of the highest plain, and found it, at 

 the height of 950 feet, capped with the usual bed of 

 gravel. 



Near the mouth of the Santa Cruz, the bed of gravel 

 on the 355 feet plain is from twenty to about thirty- 

 five feet in thickness. The pebbles vary from minute 

 ones to the size of a hen's egg, and even to that of half 

 a man's head ; they consist of paler varieties of porphyry 

 than those found farther northward, and there are fewer 

 of the gallstone-yellow kind ; pebbles of compact black 

 clay-slate were here first observed. The gravel, as we 

 have seen, covers the step-formed plains at the mouth, 

 head, and on the sides of the great valley of the Santa 

 Cruz. At a distance of 110 miles from the coast, the 

 plain has risen to the height of 1,416 feet above the 

 sea ; and the gravel, with the associated great boulder 

 formation, has attained a thickness of 212 feet. The 

 plain, apparently with its usual gravel covering, slopes 

 up to the foot of the Cordillera to the height of between 

 3,200 and 3,300 feet. In -ascending the valley, the 



