FROM THE EDDYSTONE GROUNDS TO START POINT. 377 



Section IV. 



THE BOTTOM-DEPOSITS. 



The question of the changes in the nature of the fauna which take place 

 in shallow waters with changes in the character of the bottom-deposit 

 is one to which considerable attention has recently been paid. More 

 especially Marion (No. 74) in the Gulf of Marseilles, Petersen (No. 95) 

 in the Kattegat, Herdman (Nos. 40-44) in the Irish Sea, and Pruvot 

 (No. 98) in the Mediterranean and in Prittany have dealt specially 

 with the subject. At the instigation of these naturalists chemical 

 analyses of the bottom-deposits have been made and their geological 

 characters have been described. 



In considering the relation between the nature of the bottom-deposit 

 and the fauna living upon it, the factor which appears to be of primary 

 importance is the texture of the deposit rather than its geological 

 character. This is fully recognised by the authors named, and it is a 

 point to which the attention of marine naturalists has always been 

 directed. 



The terms rock, stone, gravel, coral or nullipore, shell, sand, fine sand, 

 mud, muddy gravel, muddy sand, etc., have generally been used to 

 express differences in the nature and texture of the deposits. These 

 designations are adopted from navigators, and are the terms used on 

 Admiralty and other charts to express the nature of the material 

 brought up on the tallow placed for the purpose at the end of the 

 ordinary sounding-lead. But a very slight study of the accounts of 

 dredgings given by various authors is sufficient to show that these 

 terms are used in very different senses by different individuals. The 

 term sand especially seems to be employed in a very broad sense to 

 include deposits having a wide range of texture. Several of the other 

 terms are also much confused. 



After the present investigation had proceeded for some time it 

 seemed necessary that an attempt should be made to actually measure 

 the texture of the deposits found in different places, so that accurate 

 and consistent descriptions might be given. The readiest method of 

 doing this is to separate the particles of different sizes by means of a 

 series of sieves of standard meshes, and then to determine the percentage 

 of material of each size in the sample. Since my results were obtained 

 I have found that three or four of Petersen's samples were examined in 

 the same way, though, unfortunately, he does not give the numbers of 

 the hauls from which they were obtained, and it is impossible, therefore, 

 to discover the fauna connected with them. 



