170 PROF. J. B. HARRISON ON EXTRANEOUS [vol. lxxv, 



Resemblance in composition between the residua from the low- 

 level limestones and the soils on them is not marked, as these soils 

 are only in part sedentary, and hence their compositions are largely 

 affected by detritus received from sources other than the under- 

 lying limestone. 



In the high-level soils the proportions of calcium oxide are 

 higher, and those of phosphoric anhydride far higher than in the 

 residua. The former circumstance is of course due to some of the 

 calcium carbonate of the limestone not having been dissolved and 

 removed in solution, the latter to the accumulation in the soils of 

 part of the calcium phosphate soluble in dilute acid which is a con- 

 stituent of the limestone, its average content of calcium phosphate 

 being 027 per cent., while that of the fossil corals is only '0017 

 per cent. 



I have made analyses of twelve normal, unweathered samples 

 of Barbados coral-rock. The content of insoluble mineral con- 

 stituents in them was as follows : — 



Maximum. Mean. Minimum. 



•75 '36 traces 



but other samples that I examined contained much higher propor- 

 tions : for instance, 1*16, l - 84, 2*32, and 3 - 88 per cent, of insoluble 

 residue ; these samples, however, were not normal unaltered lime- 

 stones, the high proportion of insoluble matter in them having 

 been derived either from concentration during their weathering, or 

 possibly from the weathering of limestones on the higher ten-aces 

 while the low-level rocks were being deposited. 



The magnesium oxide found in the residua does not represent 

 all that is normally present in the coral-rock. The rock also con- 

 tains a proportion of magnesium carbonate which is largely an 

 extraneous constituent of true coral-limestone. The proportions of 

 calcium carbonate and magnesium carbonate that I found in four- 

 teen normal specimens of the limestone were : — 













Maximum. 



Mean. 



Minimum. 





CaC0 3 ... 









97-26 



98-29 



99-00 





MgC0 3 









2-44 



1-47 



•57 



l*, as 



expressed 



in 



ter 



ms 



of magnesia 



Maximum. 

 2-13 



to 100 of lime 



Mea n. 

 1-27 



Minimum. 



•47 



The proportions of the magnesium oxide are lowest in specimens 

 of the limestone consisting entirely of reef-corals, and highest in 

 specimens of lagoon-deposits. 



It may be of interest to place on record the complete results of 

 my many analyses of the Barbados coral-limestones, as very few 

 •complete analyses of such limestones are available. In order to 

 avoid the use of several decimal places, the mean compositions 

 are expressed below in parts per million or in milligrams per 

 kilogram : — 



