part 3] 



MINERALS IX CORAL-LIMESTONES OF BAIMJADOS. 



1G!> 



that the minerals would have if they had been weathered out from 

 the limestones by solution of its carbonates and thence washed into 

 the pot-hole, and therefore confirm my views as to the origin of 

 the clay. In the residua separated without the use of acid and 

 alkalies many siliceous sponge-spiculcs and some kidney-shaped 

 blebs of opaline silica are present. A very few minute blue specks 

 of vivianite (SFeO P.,0 5 SH.,0) were detected in this residuum. 

 The other iron minerals found in it are haematite and limonite, 

 neither magnetite nor ilmenite being present. The minerals in 

 the pot-hole clay are far more affected by detrition and weathering 

 than are those in the residua separated chemically from the lime- 

 stone. 



The proportions of the unweathered and partly-weathered 

 volcanic minerals, including the splinters of glass, can be approxi- 

 mately estimated from the quantities of the oxides of sodium, 

 potassium, magnesium, calcium, iron (ferrous) and titanium, shown 

 in the analyses of the various residua. The compositions are, in 

 round figures, approximately as follows : — 





! 1 



1. 2. 3. ; 4. 



5. 6. 



Organic silica (sponge-spicules). 

 Quartz and chalcedony 



2 2 ! 2 10 

 - ! - 2 7 

 67 | 37 50 57 

 17 , 29 20 14 

 14 j 32 26 12 



4 

 13 

 22 

 12 



47 



4 

 20 

 45 



8 i 

 23 ' 



Kaolin 





Volcanic minerals and glass ... 



1. Mount Misery coral; 2. High-level corals; 3. High-level beach-rock; 

 4. Pot-hole clay, Castle Grant ; 5. Low-level corals ; 6. Low-level beach- 

 rock. 



The residua, both artificial and natural, if separated from the 

 limestone without either chemical or mechanical loss, should give 

 rise to fertile soils of favourable physical texture and chemical 

 composition. Many years ago I made analyses of several of the 

 red residuary soils of the highest limestone-terraces. The mean 

 contents of important constituents of plant-food in the residua of 

 the limestones of the high-level terraces and in the soils lying on 

 them are as follows: — 



Residua from 

 High-level Pot-hole High-level 



Limestones. Soils. 



Organic matter 5*60 2*40 2*97 



N 2 -22 -1<) -20 



CaO -43 -OS 1-79 



MgO 1-12 -12 -57 



K,0 -72 -61 -12 



P 2 0, -013 -015 -23 



The other constituents of interest are : — 



Al 2 O a 25-60 23-98 21*38 



Total iron. ;is Fe.O, 14*92 13-76 12-22 



TiO a 2-16 2-:;:: 2-12 



Q. J. (i.S. No. 2 l .)\). o 



