58 Duralde's communication 



road and make a dust, which the rains carry off to these 

 places, hy which the bottom becomes insensibly raised, when 

 it dries, and is hardened by the going and coming of these 

 animals. This, if I do not deceive myself, is the solution of 

 the problem ; my opinion arises from the inspection of certain 

 low places, which appear most certainly to have been moras- 

 ses formerly; for although they are now become firm and 

 solid, yet we can still observe the places of all the tufts of 

 reeds, with the intervals which have separated them, such as 

 we find them in muddy marshes. Whatever may be the cause 

 of this elevation, the nature of the soil warrants the opinion,, 

 that it has not been caused by alluvions. 



There is also an appearance in this range of country, 

 which is very common, but which continues to surprise me 

 every time I observe it; namely, the circular form of certain 

 marshes of different diameters upon the highest grounds. I 

 have not ascertained the fact mathematically, but the eye so 

 well attests their regularity, that it seems as if art could 

 not have rendered them more perfect. These marshes are 

 not deep, and most of them dry up in times of great drought, 

 and the bottom deepens gradually from the circumference to 

 the centre. I have never observed them without endeavour- 

 ing to ascertain the cause of them ; none other, equally satis*- 

 factory, has presented itself to my mind, dian that they were 

 cavities which have been thus excavated by some whirlpool, at 

 the time the whole surface was covered by water. I cannot 

 help avowing, even at the risk of being accused of temerity, 

 that the existence of these marshes, combined with the cir- 

 cumstance of finding these bones at such extraordinary depths, 

 and also with a tradition of the Alacapas (a neighbouring Indian 

 tribe) has almost convinced my mind that such a state of things 

 existed at some very distant period. 



