INEQUALITIES OF SEDIMENTATION 357 



is vei'y considerable and varies greatly along different parts of the 

 shore line. In one area a mussel bed may have a population of 

 16,000 molluscs to every square foot/ Another area may be prac- 

 tically barren of animal life. These barren areas occur in the 

 tropics as well as elsewhere. Dr. Fred C. Baker^ has called the 

 writer's attention to a portion of the northeast coast of Brazil 

 where molluscan life is extremely scarce. 



The data which has been recorded in connection with oyster 

 culture is instructive in connection with any consideration of the 

 part which marine molluscs play in furnishing material for lime- 

 stones. It is reported^ that the oyster when introduced into a 

 Louisiana area where it had been previously unknown yielded 

 1,500 to 2,000 bushels per acre in two years. In this particular 

 case the shells of a single mollusc contributed an annual average 

 thickness of about 4 inches of calcareous material over the whole 

 of the bottom on which they lived. These figures will serve to 

 illustrate the very different rates at which marine shells contribute 

 to the materials for limestone formation. Along those parts of 

 a coast where large prohfic species like the oyster are dominant the 

 calcareous element of the sediments must increase much more 

 rapidly than where less prolific and smaller species make up the 

 fauna. 



An important factor in the accumulation of marine shells 

 in any given area is the dehcate adjustment to a particular kind of 

 bottom environment which characterizes most marine molluscs. 

 The way in which this adjustment would operate with the sand- 

 loving cockle of the English coast has been aptly pointed out by 

 Hunt. 



Were the supply of sand in Torbay to be cut off, another common feature 

 in geology might meet with an illustration, viz., the sharply defined zone in 

 which fossils frequently occur. Were such supply to be cut off (and from the 

 isolated patches of red sandstones that skirt the bay we see how much less it 

 already is than it has been in past time) the sand-loving cockles could not 

 fail to be affected, and might even become extinct in this locality at a very 

 rapid rate.'' 



^ James Johnson, op. cit. ^ Letter, June 12, 1912. 



3 Hugh M. Smith, National Geographic Magazine, XXIV (191 3), 267. 

 ^ A. R. Hunt, "Notes on Torbay," Rep. and Trans. Devonshire Association for 

 the Advancement oj Science, Literature, and Art, X (1878), 186-87. 



