372 



HISTORICAL GEOLOGY. 



curious habit of gathering bits of wood, small dead shells, 

 or even grains of sand, and webbing them together to 

 form a cylindrical hollow case in which it lives. When it 



PIG. 334. Fragment of indusial lime- 

 stone (natural size), showing the 

 caddis-worm cases. 



FIG. 335 Recent caddis- worm, 

 with its case. 



wishes to walk about, it puts out the head and legs for 

 that purpose, as seen in the figure. These cases are left 

 when the worm changes into the caddis-fly. We may 

 imagine, then, that in Auvergne, in Miocene times, there 



FIG. 336. Prodryas Persephone. (After Scudder.) 



was a lake in which lived countless generations of caddis- 

 worms, and their cast-off cases accumulated until a 

 deposit, two to three feet thick, was produced. 



