222 Gravel Formation of Patagonia, paet n. 



gravel gradually becomes entirely altered in character : 

 high. up. we have pebbles of crystalline feldspathic rocks, 

 compact clay-slate, quartzose schists and pale-coloured 

 porphyries ; these rocks, judging both from the gigantic 

 boulders in the surface and from some small pebbles 

 embedded beneath 700 feet in thickness of the old 

 tertiary strata, are the prevailing kinds in this part of 

 the Cordillera ; pebbles of basalt from the neighbouring 

 streams of basaltic lava are also numerous ; there are 

 few or none of the reddish or of the gallstone-yellow 

 porphyries so common near the coast. Hence the 

 pebbles on the 350 feet plain at the mouth of the Santa 

 Cruz cannot have been derived (with the exception of 

 those of compact clay-slate, which, however, may equally 

 well have come from the south) from the Cordillera in 

 this latitude ; but probably, in chief part, from farther 

 north. 



Southward of the Santa Cruz, the gravel may be 

 seen continuously capping the great 840 feet plain : at 

 the Eio Gallegos, where this plain is succeeded by a 

 lower one, there is, as I am informed by Captain Suli- 

 van, an irregular covering of gravel from ten to twelve 

 feet in thickness over the whole country. The district 

 on each side of the Strait of Magellan is covered up 

 either with gravel or the boulder formation : it was 

 interesting to observe the marked difference between 

 the perfectly rounded state of the pebbles in the great 

 shingle formation of Patagonia, and the more or less 

 angular fragments in the boulder formation. The 

 pebbles and fragments near the Strait of Magellan 

 nearly all belong to rocks known to occur in Fuegia. 

 I was therefore much surprised in dredging south of 

 the Strait to find, in lat. 54° 10' south, many pebbles 

 of the gallstone-yellow siliceous porphyry ; I procured 

 others from a great depth off Staten Island, and others 



