ORIGIN OF THE CATINGA LIMESTONES 193 



The accompanying photograph (see plate 16, figure 1), taken at Boa 

 Vista, 75 kilometers west of Bomfim, or Villa Nova, shows the present 

 condition of the bed of Eio Salitre at that place and in the vicinity. All 

 the flat foreground is made of a fine, soft, cream-colored limestone, over- 

 grown with a sparse covering of bushes and tall weeds. The banks of the 

 stream channel are commonly steep on one side and gently sloping on the 

 other. The entire surface of the stream channel is not so flat as this 

 photograph suggests, but there are occasional depressions, and in the wet 

 season these depressions contain water. 



The slope of the stream bed is even and gentle, and whenever there is 

 any sudden change of level it is marked by a natural embankment of 

 recently formed travertine. One of these embankments is shown in the 

 accompanying photograph. (See plate 15, figure 2.) They vary con- 

 siderably in height ; sometimes they are only half a meter high, but some 

 of those seen are 4 meters high. The one shown in the accompanying 

 photograph is a little more than 2 meters in height. The rock of which 

 these natural embankments are made is not soft like the marly limestones 

 filling most of the channels, but is rather hard on the upper face. They 

 are all more or less overhanging and cavernous on the downstream side. 

 Where the channel of the stream is not well defined, but spreads out to 

 form arms or embayments, the floors of these embayments are flat and 

 covered with the soft marly limestone. 



The explanation of this filling up of the stream channels and the for- 

 mation of these natural barriers, embankments, or dams is simple enough, 

 in so far as the method is concerned. The waters flowing from the sur- 

 rounding region of limestone are heavily charged with lime. When these 

 waters are exposed to the sun and warmed in the shallow streams, the 

 carbon dioxide is partly liberated and the lime is deposited over the floor 

 of the channel. The filling usually begins at falls or cataracts and pro- 

 ceeds both upstream and downstream. The shallower parts fill first be- 

 cause of the greater exposure of the water at those places, and later the 

 deeper pools are slowly encroached on. Any downstream slope of the 

 stream bed that causes a rippling of the water exposes it to the air, liber- 

 ates more carbon dioxide, and thus causes an increased deposition of lime 

 on the downstream side. In time these places build up the barriers as we 

 now have them. 



The newly formed limestone, however, is by no means confined to the 

 stream channels. Similar deposits are in process of formation over large 

 areas of the flat portions of the limestone valleys. As so frequently hap- 

 pens in limestone regions, the surface of the ground is more or less pitted 

 by sink-holes or depressions caused by underground solution and removal. 



