NOTES AND QUERIES. 



261 



depressions gradually increasing in size by the continuous action of tke acidu- 

 lated water, would grow in proportion to the activity or duration of the errosive 

 action. This action would also be exerted in equal portions round th.3 cir- 

 cumference of the hollow, provided that the sands or gravel above were of a 

 moderately uniform texture, and its result would be to give the pipes a more 

 or less circular, funnel-shaped, or cylindrical form, depending greatly on the 

 solidity of the chalk and the duration of the errosive agent in action. The 

 longer this action continued, the greater would be the tendency to deepen 

 vertically, or in other words, to pass successively from the "cup" to the 

 "funnel-shape" (fig. 3), and lastly to the cylindrical form presented in the 

 diagram (fig. 1). 



Mr. Prestwich explains the gradual formation of these pipes in the following 

 manner : — 



Fig. 7. — Vertical section in a more advanced state, showing where the action ceases, 

 except in a vertical direction towards "P." 



" If we divide a line drawn through the centre of the horizontal section of 

 the top of a pipe into three equal parts {A a, a b, b B, fig. 5), and carry 

 down two perpendicular lines from a and b until they meet the sices of the 

 pipe at c and d in the vertical sections (figs. 2 and 3), it is evident that in 

 (fig. 2) the relative dimensions oi Ac, c d, and d B, are very nearly the same, 

 the line c d being very little less than c A or d B ; still the difference is suffi- 

 cient supposing equal quantities of water to pass in equal time through the 

 equal widths A a, a b, b B, to make the relative quantity supplied to c d 

 greater than that supplied to A c and B d ; consequently in (fig. 3) the water- 

 wear between c d would be slightly greater (aided also by the tendency of the 

 water to converge at p) than between A c and B d, and t!ie cavity of the incipient 



