342 PRINCIPLES OF STRATIGRAPHY 



falls or cataracts, and proceeds both upstream and downstream, fill- 

 ing the shallow parts first, because of the greater exposure of the 

 water at those places. "Any downstream slope of the stream bed 

 that causes a rippling of the water exposes it to the air, liberates 

 more carbon dioxide, and thus causes an increased deposition of 

 lime on the downstream side." Barriers are thus built up in time. 

 Embankments formed in this way vary in height from half a meter 

 to four meters. The rock is hard at the upper surface, more or 

 less overhanging and cavernous on the downstream side. Where 

 streams spread out to form arms or embayments, the floors of these 

 are covered with soft, marly limestone. The same is true over the 

 flat portions of the limestone valleys. In the low grounds water 

 often remains stagnant in broad, shallow pools, especially in some 

 of the sink-hole depressions. The deposit here formed is always 

 soft and marly when fresh, but becomes as hard as a normal lime- 

 stone on ageing. "At many places these lime deposits, both the 

 newer and older ones, contain plant impressions and enclose the re- 

 mains of land shells." Sometimes the shells are cemented together 

 into a hard-shell limestone, the species being those still existing in 

 the region. Angular as well as water-worn fragments of all kinds 

 of rock are not infrequently found embedded in this limestone. 



"When a broad, gentle and rather even slope carries the lime- 

 charged waters in shallow sheets toward a channel, the lime is pre- 

 cipitated more rapidly along the edge of the plain, where, on ac- 

 count of a change to a steeper grade, the water breaks into ripples 

 or spray. This causes the bluft' to encroach steadily on the low 

 ground, and the process must eventually lead to the low ground be- 

 ing entirely filled up." The blufifs thus formed curve over at the 

 top, and are full of caverns due to the irregularity of the deposit. 

 The roofs of the caverns are hung with stalactites. "Often the 

 travertine is deposited in masses of such shape that they break 

 from their own weight and form a talus slope at the base of the 

 bluff, and this, too, in time is covered over by later deposits, and is 

 incorporated in the great limestone sheet." Some of these bluffs are 

 from 25 to 30 meters high, which is, therefore, the thickness of the 

 great limestone thus forming on dry land. The process is one of 

 chemical aggradation. 



Deposits of Lime by Springs. Calcareous tufa is a common 

 deposit by springs emerging from limestone formations. The de- 

 posit of lime here is due largely to the relief of pressure and the 

 escape of COo, which held the lime in solution (see postea). Or- 

 ganic bodies, such as leaves, sticks, etc., are commonly encrusted by 

 this lime, which at first is a fine slime, but soon hardens on ex- 



