20 NATURAL SCIENCE. ,,^«ch. 



and the bones, other than otoliths, of fishes is remarkable, and 

 indicates that hard parts containing a large amount of phosphate of 

 lime and organic matter are especially liable to decomposition and 

 solution. The siliceous organisms which contribute to the formation 

 of widely distributed deposits are radiolaria and diatoms. Sponge 

 spicules are very commonly present, but do not form an important 

 part of any of the deposits examined. Hexactinellid spicules are 

 characteristic of the deeper waters ; tetractinellid and monaxonid 

 spicules, on the other hand, occur most abundantly in the shallower 

 regions. Vertebrata are represented principally by the teeth and 

 otoliths of fishes, and the ear-bones of whales ; some of these remains, 

 including the Carcliarodon megalodon of the Crag, are believed to be 

 referable to extinct Tertiary species. More than fifteen hundred 

 sharks' teeth were obtained in one haul at a point in the South 

 Pacific. 



The mineral constituents not formed on the floor of the ocean 

 separate themselves into two well-marked groups: (i) pyroclastic 

 materials, including crystals, crystal fragments, pumice, lapilli, and 

 minute glassy particles ; (2) detrital materials, resulting from the 

 destruction of crystalline, schisto-crystalline, and epiclastic rocks. 

 The former are universally distributed ; the latter are practically 

 limited to a comparatively narrow zone round the land masses, and 

 only occur in the pelagic deposits where these are formed under the 

 influence of ice-drift, or in regions where the surface is exposed to 

 powerful winds blowing from desert regions. 



The universal distribution of pyroclastic materials, including 

 pumice, is not surprising when we take into consideration the posi- 

 tions of active volcanoes in relation to the sea and the characteristic 

 phenomena of paroxysmal eruptions. Submarine volcanic action 

 must also contribute to the supply of pyroclastic material. Frag- 

 ments of pumice varying in size from that of a man's head to that of 

 a mustard seed, and generally more or less rounded, were dredged at 

 many stations, sometimes hundreds of miles from land. The surface 

 layers, in all cases, had been more or less changed to a soft clayey 

 material — a fact of some importance as bearing on the origin of 

 abyssal clays. The three main types of volcanic rocks — acid, inter- 

 mediate, and basic — are all represented amongst the "Challenger" 

 collections. By the disintegration of these different varieties of 

 pumice, minute glassy particles are formed and minerals liberated, 

 both of which are identical with the constituents of fine volcanic 

 ash. It is not always possible, therefore, to say whether a given 

 crystal or glassy fragment has been produced by the destruction of 

 pumice or by the actions which accompany paroxysmal eruptions. 



In many regions of the deep-sea, lapilli and pebble-like fragments 

 of a basic volcanic glass were obtained. These were always found to 

 be more or less altered to a resinous, yellowish green, or reddish brown 

 substance, having all the characters of palagonite. The minerals 



