GEOLOGY OF PATAGONIA. 219 



ly occurs in the case of musquitoes — on the blood 

 of what animals do these insects commonly feed? 

 The guanaco is nearly the only warm-blooded 

 quadruped, and it is found in quite inconsiderable 

 numbers compared with the multitude of flies. 



The geology of Patagonia is interesting. Dif- 

 ferently from Europe, where the tertiary forma- 

 tions appear to have accumulated in bays, here 

 along hundreds of miles of coast we have one great 

 deposit, including many tertiary shells, all appa- 

 rently extinct. The most common shell is a mass- 

 ive, gigantic oyster, sometimes even a foot in diam- 

 eter. These beds are covered by others of a pe- 

 culiar soft white stone, including much gypsum, 

 and resembling chalk, but really of a pumiceous 

 nature. It is highly remarkable, from being com- 

 posed, to at least one tenth part of its bulk, of In- 

 fusoria : Professor Ehrenberg has already ascer- 

 tained in it thirty oceanic forms. This bed ex- 

 tends for 500 miles along the coast, and probably 

 for a considerably greater distance. At Port St. 

 Julian its thickness is more than 800 feet ! These 

 white beds are everywhere capped by a mass of 

 gravel, forming probably one of the largest beds 

 of shingle in the world : it certainly extends from 

 near the Rio Colorado to between 600 and 700 

 nautical miles southward ; at Santa Cruz (a river 

 a little south of St. Julian) it reaches to the foot of 

 the Cordillera ; half way up the river, its thickness 

 is more than 200 feet ; it probably everywhere ex- 

 tends to this great chain, whence the well-rounded 

 pebbles of porphyry have been derived : we may 

 consider its average breadth as 200 miles, and its 

 average thickness as about 50 feet. If this gi-eat 

 bed of pebbles, without including the mud neces- 

 sarily derived from their attrition, was piled into a 



