DENTAL HARD TISSUE DESTRUCTION 



108 



Fig. 2. Rock "erosion," curiously similar to certain circular erosions of 

 human teeth (insert). (Reproduced from "The Toby-jug Stone — Kannesteinen 

 near Maal0y, Norway," with the permission of Mittet Foto A/S, Oslo, Norway.) 

 In the insert is shown the ultimate "hourglass" appearance of one of the 

 mandibular incisors seen in Fig. 1 I at an earlier stage of erosion. At the time 

 of extraction, the erosion process had extended all around the lingual, labial, 

 mesial, and distal surfaces. 



