AGE OF MAMMALS. 269 



of small pebbles a few inches thick. 4. A layer of sand 

 three feet thick, containing shells called Chama and Ve- 

 nus differ mis. 5. A stratum four feet thick, consisting 

 mostly of a compact mass of Chama and Area centena- 

 ria. 6. A stratum two feet thick, mostly of large Pec- 

 7. Another stratum of Chama, with Area centena- 

 ria, Panopea reflexa, about six feet in depth. 8. A sec- 

 ond layer of Pectens, with Ostrea compressirostra, one 

 foot thick. 9. Another stratum of Chama three feet 

 thick. 10. A stratum of Pectens and Ostrea five feet 

 thick. When the shells are very perfect we know that 

 the animals that inhabited them lived and died on the 

 spot, and that the strata were formed very gradually in 

 still water. We make a different inference when the 

 shells are much broken up. On the York River, in Vir- 

 ginia, there is a porous rock, in some places forty feet 

 in height, which is made up almost wholly of comminu- 

 ted shells. Here was no still water, but there were the 

 rush of the tide and the breaking of the surf during all 

 the time that this rock was being deposited. 



379. Indusial Limestones. Another example, in addi- 

 tion to those already mentioned, of the contributions of 

 small animals in the work of rock-making we have in the 

 inclusial limestones in the ancient province of Auvergne, 

 in the central part of France. The agent in this case 

 was the larva of a species of fly, allied to the common 

 caddis-worm of the angler of the present day. We find 

 now that the larvae or grubs of different species use va- 

 rious materials for their incfusia, or covers, some gluing 

 together small bits of wood, others choosing grains of 

 sand, and others still the shells of 

 small mollusks. One of the last 

 is seen in Fig. 162. The animal 

 is in a tube which it has con- 

 structed for itself of minute shells, 

 and, living in this, it thrusts its 

 Fi ? m head and a portion of its body 



