166 
PHYSICAL HISTORY OF THE 
received notions as to the ancient character 
of the Amazonian deposits, referred by Hum¬ 
boldt to the Devonian, and by Martius to the 
Triassic period, and considered by all travel¬ 
lers to be at least as old as the Tertiaries. 
The result, however, confirmed his report, at 
least so far as the component materials of the 
formation are concerned; but, as will be seen 
hereafter, the mode of their deposition, and 
the time at which it took place, have not been 
the same at the north and south ; and this 
difference of circumstances has modified the 
aspect of a formation essentially the same 
throughout. At first sight, it would indeed 
appear that this formation, as it exists in the 
valley of the Amazons, is identical with that 
of Rio; but it differs from it in the rarity of 
its boulders, and in showing occasional signs 
of stratification. It is also everywhere under¬ 
laid by coarse, well-stratified deposits, resem¬ 
bling somewhat the recife of Bahia and Per¬ 
nambuco ; whereas the unstratified drift of the 
south rests immediately upon the undulating 
surface of whatever rock happens to make the 
foundation of the country, whether stratified or 
crystalline. The peculiar sandstone on which 
