1883.] The Naturalist Brazilian Expedition. 357 
little doubt that it was formed in the same manner ; after existing 
for some time as. an estuary of the small rivers which now flow 
into it, it was closed by the sand-banks, and its waters emp- 
tied into the ocean by the Rio de Sao Gonçalo ; the Guahyba es- 
tuary subsequently overlapped the mouth of the Sao Gongalo, and 
when this estuary became a lake the Lagoa Mirim was reduced 
to the form of a tributary. 
Owing to the shallowness of the lakes an immense evaporation 
takes place from them, and at certain seasons this nearly or quite 
balances the inflow from the rivers; salt water may then pass in 
from the ocean to the southern part of the Lagoa dos Patos, fill- 
ing the Sao Gonçalo channel, and even extending far up the 
Lagoa Mirim. The yearly outflow from the lakes, owing to the 
same cause, is very slight. This fact explains the irregular and 
feeble currents of the Rio Grande channel, and it is the real cause 
of the heavy bar at its mouth. If the outflow were strong and 
regular, or only influenced by the tides, the bar would have at 
least one permanent and deep passage through it, or at worst the 
question of removing the obstruction might be solved by means 
of jetties, 
. The Guahyba is very picturesque. High granite hills skirt 
the eastern shore, sometimes bare and precipitous, oftener with 
grassy slopes and forest-lined ravines. A few rocky islets are 
Scattered over the channel; one of them is used for a powder 
Magazine and signal station, but there are no fortifications. For 
twenty miles above the lake the channel has an average width of 
five or six miles, but at Porto Alegre it is suddenly narrowed to 
less than a mile by two opposite projecting points. The southern 
Point is low and bordered by swampy forest; the northern, on 
which the city is built, is a granitic promontory about 150 feet 
high. This elevation, slight as it is, adds greatly to the pic- 
turesqueness of the place; white and yellow-washed houses, 
with red tile roofs, are seen rising one above another, the public 
buildings crowning the hill with fine effect. 
Of all Brazilian cities this is the least tropical in appearance, 
architecture shows some traces of that degraded Roman type 
commonly seen in Spanish and Portuguese America, but even 
this 'S not very noticeable, and on landing we might have ima- 
Sined ourselves in a seaport town of the United States. The 
Streets are wide and well paved with porphyry from the surround- 
