490 



SCIENCE. 



[Vol. IV., No. 95. 



if they had been nothing more than so many 

 strings or bits of paper, is deeply impressive. 

 Forests so dense that I do not know how to 

 conve}' an adequate idea of their density and 

 gloom, are uprooted, torn, and swept away 

 like chaff; and, after the full force of the waves 

 is broken, they sweep on inland, leaving the 

 debris with which they are loaded, heaped and 

 strewn through the forests. The most power- 

 ful roots of the largest trees cannot withstand 



SKETCH ON THE ILHA DOS PORQUINHOS, SHOWING THE UPROOTED TREES. 



the porordca, for the ground itself is torn up 

 to great depths in many places, and carried 

 away by the flood to make bars, add to old 

 islands, or build up new ones. Before seeing 

 these evidences of its devastation, I had heard 

 what I considered very extravagant stories of 

 the destructive power of the porordca ; but, 

 after seeing them, doubt was no longer pos- 

 sible. The lower or northern ends of the 

 islands of Bailique and Porquinhos seemed to 

 feel the force of the waves at the time of mj- 

 visit more than any of the other islands on 

 the south-east side of the river ; while on the 

 northern side the forest was wrecked, and the 

 banks washed out far above Una Nova. 



The explanation of this phenomenon, as 

 given b}' Condamine, appears to be the cor- 

 rect one ; that is, that it is due to the incom- 

 ing tides meeting resistance, in the form of 

 immense sand-bars in some places, and narrow 

 channels in others. 



Most persons who mention the pororoca, 

 say that it breaks as far up the Amazon as 

 Macapa; and, indeed, the people of Macapa 

 themselves often refer to the rapid cutting- 

 away of the river-banks near their cit3 T as the 

 work of the porordca. It is true that these 

 banks are being rapidly cut down ; and it is 

 even a common thing to sec, in this part of 

 the country, the stilted houses — the floors 

 being nearl}' two metres from the ground — 



that were originally built one, two, or three 

 hundred feet from the water, gradually en- 

 croached upon until they fall into the stream. 

 A portion of the old fort at Macapa was, at 

 the time of my visit, about to fall, on account 

 of the land upon which it was built being 

 washed away ; but all this is the work of a 

 rapid current, for the surf of the porordca does 

 not reach Macapa, though it may reach a little 

 farther west than I have represented on the 

 map. Moreover, there 

 is a marked difference 

 in character between 

 the washing done by 

 the porordca and that 

 done by the ordinary 

 river or tide current. 

 The latter works from 

 below, and, by under- 

 mining and softening 

 the bank, causes what 

 is known through the 

 Amazon valley as ter- 

 ras cahidas, or fallen 

 banks. The land 

 falls into the stream 

 in sections of various 

 widths, and not infrequently these form tem- 

 porary terraces miles in length. These terras 

 cahidas are most common and most exten- 

 sive on the upper Amazon during high water, 

 but they ma}' be seen on a small scale at vari- 

 ous places through the valley. The accom- 

 panying diagram and sketch were made near 

 Mazagao, on the lower Amazon. From this 

 it is clear that the work of destruction goes 

 on entirely below the surface. With the poro- 

 rdca, on the contraiy, the water is dashed fairly 

 against the banks, and the earth is washed 

 away from above as well as from below, and 

 the shore is left perfectly clean. The depth to 



SECTION OF A FALLING BANK ON THE AMAZON. 



which the banks are cut shows that this disturb- 

 ance is also a profound one ; so much so, in- 

 deed, that on the north-west side of Porquinhos 

 the deepest place in the channel of the river 

 was, in 1881, close to this island, where the 

 action of the porordca was most violent. 



