28 THE DEPTH AND MAEINE DEPOSITS OF THE PACIFIC OCEAN. 



portions of the same sounding to be rapidly made ; after the washings have 

 been dried, the Schroder van der Kolk method is serviceable for determina- 

 tion by means of the refractive index. 



In the case of particles belonging to the " fine washings," often much less 

 than 0.01 mm. in thickness, a systematic microscopical examination cannot 

 be made, optical properties becoming indistinct in the case of all the minerals 

 excepting those that are highly birefringent. They can, however, often be 

 empirically determined, a little practice showing by comparison what is the 

 usual habit of one particular mineral. For instance, phillipsite is not to be 

 mistaken, even when in extremely minute crystals, its shape and general 

 appearance being characteristic. 



One has also sometimes to have recourse to chemical analysis in order 

 to ascertain the nature of certain opaque, shapeless minerals, such as the 

 oxides of iron and manganese, phosphatic concretions, etc. 



Manganese Nodules. 



We give here descriptions of the manganese nodules obtained at the 

 various stations, arranged in the order of the cruise. 



Station 4622, 21st October, 1904. Lat. 6° 21' N. ; long. 81° 44' W. ; 

 depth, 581 fathoms. 



Fragments of a large and curiously shaped nodule were obtained at 

 this station. A flat piece of rock, four inches long by one-half inch thick, 

 served as a nucleus, having a thick covering of manganese and iron oxides. 

 The rock is some altered limestone, in which the greater part of the cal- 

 cium carbonate has been replaced by silica. The coating varies in thickness, 

 being in places as much as an inch, and shows a distinct arrangement in 

 layers, some layers being richer in iron than others. The nodule is too 

 fragmentary to enable one to restore its original shape, but it may be 

 assumed that it was very likely oblong, following generally the outline of 

 the rock forming the core. There is nothing to show what is the nature 

 of the deposit in which the nodule was found. 



Station 4656, 13th November, 1904. Lat. 6° 54.6' S., long. 83° 34.3' W. ; 

 depth, 2222 fathoms. 



No deposit was received from this station, but there were a good many 

 nodules, and from the traces of deposit adhering to them it may be inferred 

 that the nodules were formed in a Rjd Clay; the station is, moreover, near 



