1834.] GEOLOGY OF PATAGONIA. 1*3 



January gth, 1834. Before it was dark the Beagle anchored in the 

 fine spacious harbour of Port St. Julian, situated about one hundred 

 and ten miles to the south of Port Desire. We remained here eight 

 days. The country is nearly similar to that of Port Desire, but perhaps 

 rather more sterile. One day a party accompanied Captain Fitz Roy 

 on a long walk round the head of the harbour. We were eleven hours 

 without tasting any water, and some of the party were quite exhausted. 

 From the summit of a hill (since well named Thirsty Hill) a fine lake 

 was spied, and two of the party proceeded with concerted signals to 

 show whether it was fresh water. What was our disappointment to 

 find a snow-white expanse of salt, crystallized in great cubes! We 

 attributed our extreme thirst to the dryness of the atmosphere; but 

 whatever the cause might be, we were exceedingly glad late in the 

 evening to get back to the boats. Although we could nowhere find, 

 during our whole visit, a single drop of fresh water, yet some must 

 exist ; lor by an odd chance I found on the surface of the salt water, 

 near the head of the bay, a Colymbetes not quite dead, which must have 

 lived in some not far distant pool. Three other insects (a Cincindela, 

 like hybrida, a Cymindis, and a Harpalus, which all live on muddy flats 

 occasionally overflowed by the sea), and one other found dead on the 

 plain, complete the list of the beetles. A good-sized fly (Tabanus) was 

 extremely numerous, and tormented us by its painful bite. The common 

 horse-fly, which is so troublesome in the shady lanes of England, belongs 

 to this same genus. We here have the puzzle that so frequently 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. Differently from Europe, 

 where the tertiary formations appear to have accumulated in bays, 

 here along hundreds of miles of coast we have one great deposit, 

 including many tertiary shells, all apparently extinct. The most com- 

 mon shell is a massive gigantic oyster, sometimes even a foot in diameter. 

 These beds are covered by others of a peculiar soft white stone, 

 including much gypsum, and resembling chalk, but really of a pumiceous 

 nature. It is highly remarkable, from being composed, to at least one- 

 tenth part of its bulk, of Infusoria : Professor Ehrenberg has already 

 ascertained in it thirty oceanic forms. This bed extends 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 ; halfway up the river, its thickness is more than 200 

 feet ; it probably everywhere extends to this great chain, whence the 

 well-rounded pebbles of porphyry have been derived : we may consider 

 (ts average breadth as 200 miles, and its average thickness as about o 



