52 



Upon these tlieoretical considerations as a basis, we may proceed to 

 make a calcnlation.* Taking the least thickness of the beds at two feet, 

 which I consider a fair average for the ordinary shell-heaps, the amonnt 

 required to cover an acre two feet deep would be 87,120 cubic feet, using 

 the United States statute acre (rz 43,560 square feet) as a basis. Admitting 

 that each person consumed one hundred echini per day, a comnuinity of 

 twenty persons would consume two thousand per day, or, in three months, 

 184,000 echini. Having taken an echinus of the largest size, dried, and 

 reduced it to coarse grains, such as those of the layer in question, I find 

 that it occupies a cubical capacity of one and three-quarters cubic inches. 

 The specimen was unusually large, not one in fifty, as seen on the shores, 

 attaining its size. Furthermore, it was not practicable for me, without 

 reducing it to dust, to make the dry fragments as compact as they are in the 

 Echinus layer; so, if there be any error in this part of the calculation, it 

 will be on the side of prudence. At this rate, it would take 988 echini to 

 make one cubic foot of the layer, and for the sake of convenience, it not 

 being likely that an estimate of 1,000 to the cubic foot will be excessive, I 

 shall adopt that number. This would give over eighty-seven millions of 

 echini to a stratum two feet deep and covering an acre. Under the circum- 

 stances previously assumed, this would be formed by a community of twenty 

 persons visiting one locality for three months in each year and eating one 

 hundred echini four inches in diameter per diem per head in a little more 

 than four Jmndred and seventy-three years. 



To forni a deposit like that at Amchitka under the same circumstances 

 would require over twenty-two hundred years. 



It would matter practically little whether one hundi'ed largo echini or 

 eight hundred of half the diameter were eaten, the contents, either of nutri- 

 ment or of solid material, in each case being about the same. The individuals 

 not containing ova are rarely found except at a depth of several fathoms. 

 They seem to enter the shallower water Avlien gravid and to retire to the 

 deeper water after discharging their eggs. This has probably some connec- 



*I must disavow any intention of proving anything absolutely by this calculation. It is merely 

 iutontlcd to give a clearer idea t-han could otherwise be conveyed of the length of time which would bo 

 occupied in forniiug such a deposit under circumstances not iu themselves improbable, and which may 

 not materially differ from those under which the particular deposit mentioned was actually formed. 



