i? 



CONDITION OP THE SHELLS IN THE TERTIARY DEP0S1TE3. 



In general the state of the shells, and their arrangement in the 

 earth, are such as to indicate their tranquil deposition at the spots in 

 which they are found. Thus the corresponding valves are very 

 often found together and closely shut. Many of the smaller shells, 

 such as Area centenaria, Area incile, Nucula?, Venericardia alticosta, 

 and Chama congregata, which are most usually found thus, are 

 often either entirely empty, or contain a small quantity of clay 

 that is quite impalpable,^indicating plainly that they have been ex- 

 posed to no violence, and that only such solid matter as could pass 

 between the edges of the closed valves had obtained access to the 

 interior. Whenever such shells, however, have been previously 

 drilled, as is very frequently the case, even with the largest and 

 thickest shells, the interior is found entirely filled with sand, clay, 

 green sand, and small fragments of shell. In most cases the larger 

 species of shells, even when their valves appear to be in accurate 

 juxtaposition, is thus filled, and in this case it cannot be supposed 

 that the contained matter has entered through the holes thus drilled, 

 since in many instances shells of considerable magnitude are found 

 imprisoned within. Such shells, no doubt, after the death of the 

 animal, remained open, or at least partially so, and received the 

 sand, clay and other materials which they contain, by the gentle 

 action of the waves. The ligament at the hinge in the mean time 

 would decay, until at length, yielding to the pressure of the accumu- 

 lating matter above, the shell, in favourable circumstances, would 

 collapse into its natural closed condition. 



The very common occurrence of the valves in juxtaposition, is 

 a striking proof, that during or subsequent to their deposition, 

 they have not been exposed to violent agencies. This becomes 

 even more remarkable in the case of such shells as the Panopea 

 reflexa, which almost in every instance is found with the valves 

 properly united. The connexion between the two valves in this 

 shell is the slightest imaginable, after the destruction of the natural 

 organic bond, and an inconsiderable force would have sufficed to 

 separate and break the valves. 



The admirable preservation of the shells in many cases is also 

 an interesting fact, and affords another evidence of the absence of 

 all violent agencies at this period. The most fragile species of 

 Natica, delicate Tellinae, Mactra Tcllinoides, the shell and processes 



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