118 REPORT—1840. | 
level with that of the harbour below the falls. It will therefore happen, 
that at the time of low water, the water in the harbour having sunk thirty- 
five feet, there will be a fall downwards at the falls to the extent of 
thirty-five feet, partly at the upper fall, but mostly (on account of the 
greater contraction) at the lower fall. As the tide flows the fall will 
diminish gradually ; and again during the ebbing of the tide from the 
bay, the falls will gradually increase until low water. So rapid and 
violent is the motion of both currents, when the tide is rising, that the 
river stream is lifted up, as it were, above the salt water, and flows away 
down the harbour, while the salt water forces itself upwards below the 
river current. The river stream only loses itself in the salt water five 
or six miles further out to sea. In the height of summer the water in 
the river is at its lowest, that is, fourteen feet lower than in the preced- 
ing case, being twenty-one feet above the harbour level; there will of 
course be a fall downwards to that extent, and this fall will continue to 
set down until the tide shall have raised the harbour-level twenty-one 
feet : at this period there will be a level and no fall either way. But 
as the tide rises the harbour-level continues to rise with it till a fall up- 
wards takes place, the measure of which is the difference between the 
two levels. The fall upwards will be greatest at high water, after that 
it will decrease until, by the ebbing of the tide, the river is on a level 
with the harbour, then the fall will be downwards, and so continue till 
low water again. 
The fall upwards seldom exceeds six feet, but owing to the irregular 
course of the rocky ravine in which the river runs, and to the great vo- 
lume of water, it is an exceedingly striking phenomenon. The river 
level is not raised more than fifteen or eighteen inches at high water ; 
but the water in The Pot is six feet above the level of the water above 
the falls. 
There is a difference of nine feet in the rise of the tide at spring- 
tides and neap-tides ; the tides rising rather more than four feet higher 
at the former than at the latter period, and falling off rather more than 
four feet also. At neap-tides, when the river is full, there will be a fall 
downwards to the extent of rather more than four feet; on the other 
hand, when the river is at its lowest, that is fourteen feet lower than in 
the preceding case, there will be at high water a fall upwards to the 
extent of nearly nine or ten feet, alternating upwards and downwards 
as before described. 
On the Geology of Madeira. By James Situ, F.G.S. 
On the Geology and Fossil Fishes of North Brazil. By Mr. Grorce 
GaRDNER of Glasgow. Communicated by J. E. Bowman, F.G.S. 
The province of Ceara, which forms a portion of North Brazil, is 
situated between the 3rd and 8th deg. of south lat., and the 37th and 
41st of west long. It is of an oblong form, its greatest length being 
