2io THE POPULAR SCIENCE MONTHLY. 



land. The silent story of the uprooted trees that lie matted and 

 tangled and twisted together upon the shore, sometimes half 

 buried in the sand, as if they were nothing more than so many 

 strings or bits of paper, is deeply impressive. Forests so dense 

 that I do not know how to convey an adequate idea of their den- 

 sity and gloom, are uprooted, torn, and swept away like chaff ; 

 and, after the full force of the waves is broken, they sweep on in- 

 land, leaving the debris with which they were loaded heaped and 

 strewn through the forests, or lodged in the very tree-tops. The 

 most powerful roots of the largest trees can not withstand the 

 pororoca, 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 pororoca ; but, after seeing 

 them, doubt was no longer possible. The lower or northern ends 

 of the islands of Bailique and Porquinhos seemed to feel the force 

 of the waves at the time of my visit more than any of the other 

 islands on the southeast side of the river, while on the northern 

 side the forest was wrecked and the banks washed out far above 

 Ilha Nova. 



The explanation of this phenomenon, as given by Condamine, 

 appears to be the correct one that is, that it is due to the incom- 

 ing tide meeting resistance in the form of immense sand-bars in 

 some places and narrow channels in others. So long as the tide 

 advances through a deep ocean, it moves freely and swiftly ; but 

 when it passes suddenly from the deep waters of the open ocean 

 to the near-shore shallows, it stumbles upon them, as it were, and 

 the waters are heaped up.* 



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 city as the work of the pororoca. It is true that 

 these banks are being rapidly cut down ; and it is even a common 

 thing to see, in this part of the country, the stilted houses the 

 floors being nearly two metres from the ground that were origi- 

 nally built one, two, or three hundred feet from the water, grad- 

 ually encroached 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 



* Prof. Hartt attributes the porordca of the Rio Mearim in Maranhao to the form of 

 the channel. It can not be questioned that the form of the channel may modify, and does 

 modify, the force with which the surf strikes the shore ; but the single fact of its great 

 violence along the shores between the Araguary and Cape North, where the whole coast is 

 exposed to the open sea save for the protection offered by shallows, is sufficient to show 

 that form of channel is not its sole cause. 



