1834] GIGANTIC SHINGLE BEDS. 175 



sea), and one other found dead on the plain, complete the 

 list of the beetles. A good-sized fly ( Tahanus) 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 common shell is a massive 

 gigantic oyster, sometimes even a foot in diameter. These 

 beds are covered by others of a peculiar 50ft 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 its average breadth as 

 200 miles, and its average thickness as about 50 feet. If tliis 

 great bed of pebbles, without including the mud necessarily 

 derived from their attrition, was piled into a mound, It 

 would form a great mountain chain ! When we consider 

 that all these pebbles, countless as the grains of sand In the 

 desert, have been derived from the slow falling of masses of 

 rock on the old coast-lines and banks of rivers ; and that 

 these fragments have been dashed into smaller pieces, and 

 that each of them has since been slowly rolledf, rounded, 

 and far transported, the mind is stupefied in thinking over 



