SIXTH REPORT OF TPIE DIRECTOR I909 1 67 



erosive agencies. This feature of erosion is both peculiar and im- 

 portant enoug'h to receive a distinct name, and the name dentpit is 

 hereby proposed. 



Dentpits defined. Dentpits are vortex-foTmed, shallow rock 

 •concavities whose width greatly exceeds .their depth and whose 

 axes are perpendicular to the rock surface. Their diameters are 

 usually between i and 5 centimeters, and a separate dentpit has 

 much the same form and smooth character as the concave surface 

 of a watch crystal. Sections along their axes give curves that are ap- 

 proximately arcs of circles whose radii are usually between 3 and 4 

 centimeters in length. These dentpits when confluent often pre- 

 sent an appearance much like that of hammered brass, but their 

 intersections are usually very sharp and clearly defined. Like cup- 

 holes they are sometimes seen to be arranged in definite lines which 

 take the direction of the water flow. 



The differences between dentpits and cupholes are both numerous 

 and well marked. Cupholes present hyperbolic or parabolic outlines 

 on longitudinal sections. The former are usually between 5 and 15 

 centimeters in diameter and their depth is usually greater than their 

 width ; their axes are always vertical even though cut on the sides 

 of steeply sloping surfaces, and they are cut or deepened directly 

 downward ; they usually contain some fine silt in their deepest por- 

 tions. Dentpits, on the contrary, present circular outlines on per- 

 pendicular sections. They are usually between i and 5 centimeters 

 in diameter and their depth is very markedly less than their width ; 

 their axes are confined to no one direction and their cups may be 

 deepened even directly upward ; no silt ever remains in their de- 

 pressions. Dentpits are developed on rather pure calcareous rocks 

 where the water is strongly agitated by frequently broken and re- 

 flected wave motion and where the water carries but little matter 

 in mechanical suspension. 



Formation of dentpits. When quiet pure water rests against 

 limestone, molecules and ions of the latter become detached and 

 pass into the water; this process we call solution. The presence 

 of such molecules and ions in the film of water next the rock inter- 

 feres with further detachmicnts, an equilibrium is soon attained 

 and solution ceases ; the water is said to be saturated. If the water 

 has a rapid molar motion it will produce a more rapid solution of 

 the limestone because it never allows the film next the limestone to 

 become saturated, or continually removes it after saturation. The 

 more rapid the movement of the water the more rapid the solution, 



