156 



DEPTHS OF THE OCEAN 



calcareous remains, or form irregular concretionary masses, 

 though the commonest form is that of more or less rounded 

 nodules (see Figs. 134 and 135), which at any one station have a 

 general family resemblance and differ in form and size from 

 those taken at another station, looking like marbles at one 

 place, like potatoes or like cricket balls at other places. Gener- 

 ally the nodules are concretions formed around a nucleus, con- 



FiG. 126. — Tooth of Carcharodon megalodon. 

 "Challenger" Station 281, South Pacific, 2385 fathoms. 



sisting of a shark's tooth or whale's earbone, or portions of teeth 

 or bone, a piece of pumice or fragment of volcanic glass, etc., 

 though sometimes no nucleus could be detected. These nodules 

 of iron and manganese are classed with the impure variety of 

 manganese known as wad or bog manganese ore, and the 

 greater part of the manganese and iron is believed to have been 

 derived directly, along with clay, from the alteration of the rock- 

 fragments and mineral particles containing manganese and iron, 

 especially of those of volcanic origin, which are spread over the 



