July 26, 1907] 



SCIENCE 



121 



ground by means of sink-toles. These are 

 pits, commonly funnel-shaped, formed hy the 

 enlargement of crevice or joint by percolating 

 water, or by the breakdown of some portion 

 of the roof of a cave. (" Elements of Geol- 

 ogy," by William Harmon Norton, p. 46.) 



8. Underground caves sometimes give rise 

 to topographic features which are of local im- 

 portance. When the solution of the material 

 in a cavern has gone so far that its roof 

 becomes thin and weak, it may collai)se, giving 

 rise to a sink or depression of the surface over 

 the site of the original cave. This is so com- 

 mon that regions of limestone eaves are often 

 affected by frequent sinks formed in this way. 

 They are a conspicuous feature of the land- 

 scape in the cave region of Kentucky, and are 

 well known in many other limestone dis- 

 tricts. They are known as limestone sinks. 

 (" Geology," Chamberlain and Salisbury, Vol. 

 I., first ed., p. 216.) 



9. Sometimes the ceiling [of caves] gives 

 way, forming the funnel-shaped " sink-holes " 

 or " lime-sinks " so familiar in some of the 

 Mississippi valley states. (" Soils," E. W. 

 Hilgard, p. 41.) 



Statements 1, 3 and 4 account for sink-holes 

 entirely by solution. Statements 2 and 5 im- 

 ply that they are due to the collapse of the 

 roofs of caverns. Statements 6, 8 and 9 

 plainly say that they are due to the collapse 

 of cavern roofs. Statement 7 teaches that 

 some sink-holes are formed wholly by solu- 

 tion, while others are formed by the collapse 

 of cavern roofs. 



The writer thinks that he is not mistaken 

 in stating that the common idea of sink-holes 

 is that they are due to the falling in of the 

 roofs of caverns. That sink-holes are some- 

 times so formed is certain, but that this 

 method of formation is the rare exception and 

 not the rule becomes evident from the follow- 

 ing common features of such depressions : 



1. Their almost universal funnel shape. 



2. The absence in them of coarse debris 

 such as would be derived from the collapsed 

 roof. 



Caverns are irregular in shape. The sub- 

 sidence of the roof, therefore, would produce 

 in nearly all cases a depression of irregular 



outline. Of many sink-holes the writer has 

 seen, but one is recalled that was irregular 

 in outline instead of being circular or of a 

 form closely approaching a circle. In this 

 single exception, the roof of the cavern had 

 been of sandstone, and the outline of the 

 " sink " was very irregular. In the bottom were 

 large amounts of debris from the former roof. 

 The limestone had been dissolved out beneath 

 the sandstone roof until the latter could no 

 longer support itself, when the collapse took 

 place. Sink-holes of this character would be 

 expected in regions containing limestone 

 caverns if the surface rock above the lime- 

 stone were thin and consisted of sandstone, 

 or some other insoluble material. But regions 

 where sink-holes are common are those in 

 which limestone is the surface rock. 



In no case except the one mentioned has 

 the writer observed coarse debris, such as 

 would be derived from the roof of the cavern, 

 in sink-holes. In course of time such debris 

 would wholly disappear by weathering, but if 

 it had ever existed a portion of it would be ex- 

 pected to remain, in many, if not in the ma- 

 jority of cases. 



The claim is not made that limestone roofs 

 of caverns never collapse, for it is reasonable 

 to suppose that they sometimes do. But cer- 

 tainly such collapses are rare. The rule in 

 the formation of sink-holes is that they are 

 the result of solution at the surface. Their 

 locations are determined by crevices in the 

 limestone, that permit the localization of 

 the downward-moving water along tube-like 

 passages that are more or less vertical. The 

 water near the surface, in moving toward this 

 tube, enlarges the upper end by solution, 

 forming a small, funnel-like depression. This 

 depression invites more drainage, resulting in 

 a greater amount of solution and the enlarge- 

 ment of the funnel. 



Sink-holes sometimes open into caverns 

 below. The entrances to caves are sometimes 

 at the bottom of these depressions. But prob- 

 ably in the majority of cases there are no 

 caverns of considerable size immediately con- 

 nected with the sink-hole. The tubular drain- 

 age course may pass into a cavern some dis- 

 tance away, or issue at the surface as a spring. 



