208 



ROCK DISINTEaEATION AND DECOMPOSITION" 



soils of Grenada/ the most southerly of the Windward Islands. 

 The entire island is volcanic, beyond the limits of the glacial 

 drift, and offers excellent opportunities for studies of this 

 nature, the rocks (mainly andesites and basalts) being decom- 

 posed to depths of over one hundred feet. Analysis of horn- 

 blende- and hornblende-augite andesites and soil derived through 

 their degeneration, yielded Dr. Harrison as below.^ 



Analyses op Fkesh and Decomposed Andesites from Grenada 



Silica (SiO.) .... 

 Alumina (AlgOs) . . 

 Ferric Iron (PeaOs) 

 Ferrous Iron (FeO) . 

 Manganese (MnO) . . 

 Lime (CaO) .... 

 Magnesia (MgO) . . 

 Potash (KoO) . . . 

 Soda (NaaO) .... 

 Phosphoric Acid (P2O5) 

 Igiiltioii 







w N 



^. 1 



< 







■"^ S 9 

 ^ ^ w 



W S3 5 

 § g 



P5Jj EH 



ercentag 

 Each Con- 

 stituent 







(^ N 



fU^ 



62.74 



33.98 



48.69 



22.37 



13.67 



24,89 



3.38 



75.13 



3.39 

 4.35 



16.981 

 2.64/ 



o.ooj 



100.00} 



0.42 



0.28 



0.30 



27.45 



6.01 



2.48 



4.99 



17.05 



1.74 



2.28 



0.80 



64.15 



1.23 



0.26 



1.12 



8.42 



4.26 



1.35 



3.69 



13.13 



0.18 



0.14 



0.12 



31.82 



2.02 



1466 



0.00 



100.00 



100.00 



99.823 



63.09 





f4 



IS 



P > 

 oa 



77.63 

 24.87 



0.00 



72.55 

 82.95 

 45.85 

 91.58 

 86.87 

 68.18 

 0.00 



Perhaps the most striking feature of these calculations is the 

 unusually large percentage of silica lost. Alumina has also 

 been carried away in quantities above the average. The total 

 loss, amounting to 63.09% of the entire rock, is not in excess of 

 that noted in other cases, but doubtless approaches the maximum 

 amount, the available analyses showing a larger percentage in 

 but a single instance, that of the Chatham, Virginia, diabase 

 just noted. 



Weathering of Ultra Basic Rocks. — The ultra basic rocks, — 

 peridotites and pyroxenites, — from the very nature of their 

 composition, must yield on decomposition residues poor in the 



^The Eocks and Soils of Grenada and Carriacou, and the Agricultural 

 Chemistry of Cacao, by J. B. Harrison, London, 1896. 



^ The present writer has taken the liberty of recalculating these analyses 

 on the assumption that the ferric iron remained constant, rather than the 

 alumina, as assumed by Mr. Harrison. 



3 The original analysis gives also CO3 0.146%, SO3 0.038%. 



