president's address. 15 



(Plates i. and ii.) the northern wall being nearer to the 

 Hawkesbury carries tlie 100-fathom line five miles further sea- 

 ward than does the southern. Consequently a sharper angle, a 

 more abrupt fall occurs in the former than in the latter. All 

 these features of the shelf accord with the hypothesis that 

 its margin is built up by the current. In the shallower 

 -depths the rocks are swept bare of sediment : perhaps this 

 zone even suffers erosion; deeper, I suppose, that the stream 

 .sweeps detrital matter about till on reaching the edge it is 

 washed over into still water^ there to form a talus slope. From 

 this point of view the depth of the bank is an index to the 

 depth of the stream, namely, a hundred fathoms. Bej'ond the 

 shelf in the open sea, the current may run deeper still. As yet 

 the current has not been plumbed by hydrographers, if we 

 except the temperature section drawn by the Challenger Expe- 

 dition. Taking the isotherm of 65° as the current boundary, 

 tlieir diagram (opp. p. 467) carries it down to 70 fathoms. 



From eighty to three hundred fathoms according to my 

 experience, and in four hundred fathoms according to the 

 " Challenger" observations, there extends a deposit of glauconite 

 sand and mud. On washing dredgings from those depths, the 

 water is suffused with a green cloud which is slow to settle. As 

 Dr. Flint remarks, this colouration must be due to extremely 

 minute and amorphous particles.* These deposits are character- 

 istic of steep and exposed coasts like ours, where no large rivers 

 pour out detrital matter. Probably the glauconite extends 

 vertically from a hundred to a thousand fathoms and horizontally 

 along the whole coast of New South Wales. 



Messrs. Lee and Collet have traced out a complicated evolu- 

 tion for this curious marine mineral. Empty shells of foraraini- 

 fera fill with clay, the alumina of which is gradually replaced by 

 peroxide of iron, forming an internal cast. This change is 

 indicated by the colour passing from grey to various shades of 

 brown. Finally, glauconitisation ensues through the introduc- 



Flint, Bull. 55, U.S. National Museum, 1905, p. 14. 



