RIO NHGRO, PATAGONIA. 609 
racy, the irregularities here referred to. They must have resulted, as 
in the other cases referred to, from the action of different currents 
(tidal or others) removing, in places, the sand before deposited, and 
following the removal by other depositions. 
We were informed that near the town of Rio Negro there is a thin 
layer of the formation, which is an impure limestone and is burnt for 
lime. 
The lowermost layers abound, at some places, in shells of the large 
Ostrea patagonica and Pecten patagoniensis, besides other species 
mentioned by D’Orbigny and Darwin. This Ostrea attains a length 
of eight inches or more, and a thickness of two inches. Many had 
the valves united, while others were separated and broken; and some 
had been perforated by Lithodomi or Annelida before having been in- 
humed. In the upper layers of the bluff 1 found the mould of a 
Bulla, a Marginella, and a ‘Turbo. 
These tertiary beds, of which only a coast section was seen by us, 
are stated by Mr. Darwin to extend continuously eleven or twelve 
hundred miles, north and south ; in some places the formation reaches 
to the Andes, rising westward to a height of three thousand feet above 
the level of the sea.* 
* « The Patagonian tertiary formation extends continuously, judging from fossils alone, 
from Santa Cruz to near the Rio Colorado, a distance of about six hundred miles, and re- 
appears over a wide area in Entre Rios and Banda Oriental, making a total distance of 
eleven hundred miles; but this formation undoubtedly extends (though no fossils were 
collected) far south of the Santa Cruz, and, according to M. D’Orbigny, one hundred and 
twenty miles north of Santa Fe. At Santa Cruz we have seen that it extends across the 
continent ; being on the coast about eight hundred feet in thickness (and rather more at 
Saint Julian), and rising with the cotemporaneous lava streams to a height of about three 
thousand feet at the base of the Cordillera.” Darwin, South America, p. 119. 
153 
