198 



The different varieties of clay have been noted and mapped, and 

 the horizons richest in iron carbonate nodules are specially indicated 

 on the maps. 



(3.) The marls of the Tertiary Series from the well-known " black 

 soils " of the cane plantations, they form excellent soils both from 

 a chemical and mechanical point of view, the advantages they possess 

 being a considerable percentage of lime and other bases, distinct 

 porosity as compared with the clays, and the fact that they usually 

 form undulating ground with many small hills and hummocks which 

 are easily drained. The action of the lime is twofold ; it seems to be 

 requisite as a chemical constituent in all good cacao soils, and its 

 solution and removal by percolating water tends to lighten and keep 

 porous the soil which might otherwise be rather too heavy and imper- 

 vious. Some of the best cacao in Trinidad is grown on these marls. 



It is evident that to manure such soils with lime is merely a 

 waste of labour, while forking round the trees may also be quite 

 unnecessary, but basic slag supplying phosphate, and ammoniacal or 

 farm-yard manure supplying nitrogenous compounds, may form 

 valuable additions to the soil. 



The outcrops of the marls have been mapped in every area tra- 

 versed during the course of the geological survey. 



Districts in Savana Grande, between Princes Town and Poole, 

 afford instance of cacao estates on the marls. 



(4.) At many horizons in the Tertiary series rapid and minute 

 alternations of fine sands and clays are found. Such rocks on dis- 

 intigration form soils which are admirable as regards mechanical 

 condition, being neither too light nor too heavy, and draining readily. 

 The clay supplies bases (lime, "alkalis, ete.) while the sand renders the 

 soil porous and supplies silica and a proportion of iron. Fossiliferous 

 bands here and there may supply lime and phosphate in appreciable 

 quantity, but the proportions of these as a rule will be much less 

 than in the heavier soils. Between Manzanilla and the Cunapo and 

 Sansrre Grande rivers, especially in the northern part of the area 

 indicated, bands of this nature are frequently observed, and on 

 Mayaro bay areas of similar material are common, and good cacao is 

 grown on them, but it has proved impossible to differentiate the out- 

 crops during the mapping, though their occurrence in many places 

 have been noted on the maps. Such strata have been coloured on the 

 General on,. -inch map of the Eastern Coal-District as either sand or 

 clay according to the predominating constituent. 



Both basic slag and farmyard manure will probably be beneficial 

 to soils formed from these rocks. 



(5.) Where the beds of Tertiary sandstone crop out the soil is 

 likely to prove too light for the best results, and accordingly we find 

 very little cacao grown on sandstones. Some of the sandstones 

 Indeed are so pure that it would be impossible to grow cacao or bois 

 immortel on them, but others with a calcareous cement, with an 

 admixture with clay, or with fossiliferous shelly bands, furnish a soil, 

 which, if lighl is still capable of growing cacao. Where there is an 

 admixture with clay a soil admirable in mechanical condition may be 

 formed, but it may be detlicieiit in lime, alkalis, phosphates, etc. 

 The '..Meat porosity i> also a disadvantage; where the sandstones form 



