8 ME. L. OWEN ON THE [vol. lxxix, 



inches of the coral bottom. Here a rapid increase in the per- 

 centage of calcium carbonate occurs, together with a corresponding 

 decrease in the percentage of tricalcium phosphate. The organic 

 matter may either increase, remain constant, or decrease to a 

 small extent. The lime percentage remains sensibly constant, 

 with the exception of a small increase in the first few inches down 

 from the surface. The changes cited above are illustrated by the 

 accompanying graph (fig. 1, p. 7). 



The coral lying directly below the deposit is almost completely 

 dolomitized, containing usually from 43 to 45 per cent, of 

 magnesium carbonate. 



The foregoing remarks deal with the variation of the composi- 

 tion of the phosphate from surface to bottom at any one point. 



The variation in composition of the phosjmate from point to 

 point, on the surface of the island, has yet to be con- 

 sidered, and this variation may be summarized, from the results of 

 many hundreds of analyses, as follows : — 



(1) Analyses show that, throughout the deposit itself (apart 

 from the black soil) the extreme variation in phosphatic content 

 in different parts of the island is between 79 and 92 per cent, of 

 tricalcium phosphate. 



(2) The variation in composition with depth rarely amounts to 

 more than 1 per cent, in the body of the deposit at any one 

 locality ; it is sufficient, therefore, when considering the variation 

 from point to point on the island, to take the average composition 

 from surface to bottom at a single spot. 



(3) In general, on proceeding from any place on the coast 

 towards the centre of the island, the phosphate content of the 

 deposit increases, while at the same time the deposit itself increases 

 in thickness. 



In detail, however, the distribution of the phosphate according 

 to the content of tricalcium phosphate is found to follow a law 

 which is elucidated by the co-ordination of the analyses. 



(4) If Ave plot the results of the analyses in the form of ' iso- 

 phosphatic lines,' these lines are found to cut the surface-contours 

 of the ground in a perfectly regular fashion. These lines may be 

 regarded as the outcrop of what can be called ' isophosphatic 

 planes' in the deposit, and the excavations (up to the end of 1914) 

 show that such planes are persistent throughout the Avhole of the 

 phosphatic capping of the island, so far as it had been explored 

 either in workings or in pits made for the purpose of test. These 

 planes are not sharply marked off one from the other, the phosphate 

 content varying quite gradually from 79 to 92 per cent. They are, 

 ■of course, only recognizable where they cut the phosphate deposit, 

 and not where they cut the unaltered coral base. 



(5) A convenient plane for reference is the isophosphatic plane 

 of 80 per cent, of tricalcium phosphate, which cuts the sea-level 

 about 1/4 mile north of the southernmost point of the island. The 

 line of intersection of this plane with sea-level is orientated east- 



